There are a few things in this world that simply cannot be stressed enough in one lifetime. Things like "Don't phone friends at one in the morning", and "Arsenic isn't for ingestion", sometimes even things like "People who substitute the word "love tunnel" for "vagina" should be taken out and slapped around for a good five minutes before given permission to continue on." But, out of all of these, the most important thing is: "Never, ever, ever take writing structure for granted."

That being said, I'll introduce this first section. In my opinion, there are three crucial elements to writing structure.

Writing structure isn't necessarily crucial to a great plot, or great characters. You could have a regular Jack Sparrow all laid out in size 8 Garamond font, but the point still remains that people are going to hate you when they read it, simply because it's barely legible. The structure should be done well enough for people to read through it easily, as opposed to translating it.

That being affirmed, let's move on.

Let's face it, these are the ones that suck. They're a pain to watch out for, and a pain to correct, but if you manage to trudge through them, it pays off. Let's kick things off with an example, shall we? Which of the below passages translates easier?

It seems unbelievable, but mark my words, I have seen far worse when it comes to fanfiction. Again, it is not necessarily the character or the emotion, but the technicalities. There is punctuation missing, the tense changes erratically, and there are spelling and grammatical errors. Although the message can be painfully gleaned from a piece along these lines, it doesn't give the reader the satisfaction he or she is looking for. The same piece is below, with corrections.

No, it's not Shakespeare, but wouldn't you rather enjoy reading it than the previous example? The tense has been straightened out, the spelling, grammatical, and punctuation errors fixed. The process is fairly simple, provided English is a first language, and punctuation isn't completely foreign.

Spelling:
There isn't much I can say here, besides stress the importance of either a spell checker, or a beta reader that has a spell checker. Too long have I traversed the wide open ranges of fanfiction and found authors who are zealous enough to bang out two paragraph "chapters" in thirty seconds, but are suddenly not devoted enough to their work to check for spelling errors. This is sloppy. It shows a slavish concern for your work and what people think, and gives the impression that you don't really want to be putting in the effort, you're just looking for some sugar coated reviews. If you're serious about becoming a writer, you should know that editors and publication houses don't sugar coat their reviews. And if it looks as though you don't care, they don't care.

Grammar:
This is the stuff that kindergarten is made of. Anyone who protests that their story is too important to be slowed down by such mean necessities should be smote, and smote quickly. Since the subject is far to general to go through and explain, I'll go over the several mistakes that seem to reoccur the most.

-"They're" is a contraction of "they are." As in "They're setting sail on the Black Pearl tonight."
-"Their" is possessive. As in "They do it every Christmas. It's their little tradition."
-"There" is a reference to a place. "I keep my diary hidden there." Or "There's your money!"
These are the easiest to comprehend. As an additional bonus, they're quite similar to the "your" rules below.

-"You're" is a contraction of "you are." As in "Holy smokes! You're Jesus!"
-"Your" is possessive. As in "Your disciples are getting out of hand."
These can be quite ridiculous when misused. A conceivable crisis: "You're nuts!"

-"Who's" is a contraction of "who is". As in "Who is that manly, scruffy looking ranger."
-"Whose" is possessive. As in "That Hobbit, whose ring is magic, has the largest neck I've ever seen."
These are messed up easily. I've even met some authors who don't realize the word "whose" exists.

-Contractions must have apostrophes. All the time. Especially if you're writing a character that slurs his words. No one knows what a "Wedn" is, but people may be able to guess what a "Wed'n" is, if you insist on being overzealous. Think of all the apostrophes that Hagrid goes through. Considering his dialect, they're a saving grace.

There is also a side-note that I will add for the Newsie Fandom (I play favourites).
There's a concept that has been thoroughly raped by Newsie Authors called "Newsie grammar." It springs into use whenever a newsie is speaking, and makes him say things like "I'se sellin' the papers" or "I always lands on my feet". This is fine; it pertains to character. What is not fine is overkill. "I'se is" is not fine, considering "I'se" is the contraction of "I is", and therefore you're having your newsie say "I is is." The same rule applies for "We's" and "You'se.", if you insist on using them in those forms (keep in mind that "we is" and "you is" is a lot easier on the eyes.) And for the love of all that is holy, use apostrophes, as it is written above.

Punctuation:
Not many readers are sticklers for things like "only commas at the end of sentences unless it's an exclamation" or "two spaces after a period." I, myself, feel like quite the hypocrite lecturing authors about punctuation when it's one of my weakest points. But, let's face it, many enjoy being able to read through a paragraph without having to pull the sentences apart like a toffee experiment gone horribly wrong. Again, the most common mistakes, briefly summarized:

Commas.
They are not periods; they are commas. They can be used to offset sections, or in between options and adjectives.
"The sword was beautiful, long, slender, and silver with elegant, ornate, Elvish markings in the blade."
"She was small and wiry, really quite crudely shaped, but rather lovely in a childish way."
Like above, they should be used at the end of a quotation, unless it is an exclamation. (In which case, an exclamation mark would be used.)

Colons and semi-colons:
These are not like toys that can be inserted anywhere, as I've seen it done. They do have specific purposes. A colon is usually used to introduce a list of options, or as an introduction to an explanation or definition. As in:
"Here are the things ye must do to become a real pirate: first, ye must find gold. Then, ye must bury gold fer no good reason."
"I'll tell ye what a pirate does, lad: A pirate finds gold. Then, a pirate buries…"
A semi-colon is a little bit trickier. It is used to join two sentences that may have some relation. For example, the sentences "Will is a blacksmith. Jack is a pirate." are unrelated. But add a semi colon and…presto! "Will is a blacksmith; Jack is a pirate." they are relevant to one another in some way that will be determined later on in your wonderful fic.
Another, more common way of using semi-colons is to join two sentences with both the semi-colon and a transition. For example: "Will is a blacksmith; however, Jack is a pirate."

Exclamation and question marks:
These are wonderful if you want more emphasis in a quotation (generally, exclamation marks in the narrative are rather cheesy), but use more than one and it cheapens the look of your story. If you're chatting on AIM and find out that your "main squeeze" is "mackin' on" that "ho" in social studies, it's perfectly acceptable to let loose with a wild reign of exclamation and question marks that could terrorize a small Latvian army. However, if the same thing happens to your main character, one is sufficient. And, as a note to those who are overzealous, surrounding a word with exclamation marks is !!not cool!!. It makes it look as though the text is going to be used as a layout as opposed to a work of fiction.

Tense:
This is quite simple, once you get the hang of it, considering it's evident in every day speech. I have yet to meet someone who is capable of speaking English and routinely says "I went to the store and I buy things." without realizing that they sound idiotic. Therefore, don’t make your characters speak this way. And even more important: Don't let the narrative turn out this way either.

Past tense:
This is the most common tense used in story telling. It is the tense in which things have happened.
"Ash Ketchum felt the pokeball close in his hand, and sighed before sliding it into his belt. He had lost."

Present tense:
This is not quite as common as past tense, but gives the story an interesting edge. It is the tense in which things are happening.
"Ash Ketchum feels the pokeball close in his hand, and sighs before sliding it into his belt. He is losing."

Future tense:
This is an extremely rare tense to have an entire story in. It is the tense in which things will happen.
"Ash Ketchum will feel the pokeball close in his hand, and he will sigh before sliding it into his belt. He will lose."

There should be one tense in which the story should be told, and it should not be switched with others. It's perfectly fine to explain what will happen while narrating in past tense, or what has happened while narrating in present tense. But if you find yourself starting out with past and ending with present, you've done something wrong. Fix it. Now.

Point of view:
This usually isn't much of an issue with competent authors, with the exception of a story that has been edited from first person to third, with a few blunders left behind. But I find that this is the most confusing element to slog through if it gets mussed, especially when it gets as mussed as I've seen it.

First person:
A common P.O.V. This is when the story is being told from the eyes of a character.
"I could sense no recognition in her cold blue eyes, no spark of irritation, no flash of anger. She didn't know me."

Second person:
This P.O.V. is less common, but it certainly gives writing an edge.
"You could sense no recognition in her cold blue eyes, no spark of irritation, no flash of anger. She didn't know you."

Third person:
In rivalry with first person for commonness. This is when the story is being told by a narrator.
"He could sense no recognition in her cold blue eyes, no spark of irritation, no flash of anger. She didn't know him."

It's common, especially in fanfiction, for the P.O.V. to switch between character to character. This can be done in any of the above P.O.V.'s, as long as it's done well. The most confusing thing for a reader to try and comprehend is erratic P.O.V. changes with no indication of whose it is. Make sure that the switches are well defined, and the character controlling it is identified as quickly as possible, either by indication in the plot, or simply a title.

This is also a huge element. Occasionally, a missed paragraph or strange cut off can be forgiven. What is difficult to forgive is huge blocks of text that pile up on fiction pages because the author was too lazy to use the enter/return key. I'd like everyone to please place their hands on their keyboards, and examine the position of the smallest finger on your right hand. As you can see, it is exactly two centimeters from the enter/return key. It's an arduous journey, I know, but it must be made for the good of fanfiction and mankind.

Paragraphs:
These are quite simple: A paragraph is made when either a different character begins speaking, so you don't get something like this:
"What's wrong?" "Nothing." "Tell me!" "I'm pregnant by Immaculate Conception." "Oh."
It's confusing and messy, to be frank. No one knows who is speaking, and sometimes the character gets blurred in with the other when the reader starts scanning.
A paragraph is also used to introduce a new concept, either in speech or the narrative. When in speech, a new quotation mark must be added. It is used to break up the formatting, so the readers know that something new is happening.
Usually paragraphs are indented, but in the case of fanfiction.net, most indentations are erase whilst uploading. The same thing, lately, has been happening to breaks over one line long, so be careful about identifying a scene change. However, an indentation is basically the act of pushing "tab" before you begin writing.

Breaks:
I have never read a fiction or novel where breaks haven't been used. It would be like watching a movie with no cuts or scene changes (although I believe Russian Arc does it, a film that I must see sometime.) Breaks are perfectly fine, as long as they are done well.
The best way, I find, to get a continuous flow in ones story is to have longer scenes. Reading something that switches between scenes every few sentences can get some readers irksome. This is fine for certain sections of a fic, or flashbacks, but I suggest strong caution when doing so. There's only so much of it that can be absorbed.
Make sure that your breaks between scenes are reasonable. Have the next scene be chronologically close to the last, if not happening at the same time. There is nothing that negates the brilliance of a scene so much as a little authors note explaining that the rest of the action is cut out, and we're skipping to "three months later." If there is important physical action or emotional climaxes/development, for the love of Jesus, write it down. We want to read it.
Pertaining to a note in "paragraphs", those on fanfiction.net may find, upon uploading their files, that breaks larger than one line have been deleted. Therefore, it may be necessary to substitute a symbol or dash in between scenes. But again, use these with caution. Some may find it cute to type out rows of asterisks, exclamation marks, dashes, addition and @ symbols, but to anyone else, it's just annoying. Scratch the text graphics, please.

This topic is very general, considering the structure and style of sentences range in technique from author to author. In this section I can only give a few tips that are generally agreed upon.

Sentence Fragments:
Sentence Fragments are exactly what they sound like. An example follows with the fragments bolded:
"His fingers were long and thin. The fingers of an artist."
The best way I can describe them is as sentences that don't lead to anything, stating a fact and just leaving it there. These are controversial. They can be used in a certain way to encourage a dreamy, fantasy sort of atmosphere, in which the narration is broken up into short pieces. It's a romantic way of writing, and gives the fic a certain flavour.
However, as is the theme with the majority of problems in writing style, once overused this can be irritating. If you are going to use sentence fragments, be very careful about how you place them and where. A good way to see if they're working out, is to write out a draft, take a break for a few days, come back to it and read it over again with a fresh perspective. You catch subtle things you never would have caught before, and it gives you a better idea of how the fic reads.

Run on sentences:
Run on sentences are used often in freewrites and poetry. They are basically sentences that seem to go on forever, like below:
"Jack Sparrow had the type of face that Elizabeth had never seen before and knew she never would again, the jagged angles, the contours of his lips, the slant of his eyes, the deep gold of his skin that seemed to glow in the Jamaican sunset, he was the most beautiful man she had ever seen, and in an instant all thoughts of Will flew from her mind, shallow and girlish, she knew that she could never love another, for his eyes seemed to capture her soul in ways that she had never felt before, ways that she had never dreamed of, she knew no one would understand."
Unlike sentence fragments, the general consensus on these is that they're difficult. Again, it depends on how they are being used in the fic, but a stream of consciousness is difficult to wad your way through, especially in the narrative. Use run on sentences at your own discretion, and make absolutely sure that they are consistent with point of view and tense.

I assure you that the examples above have not been altered or exaggerated to stray, in any way, from their true forms. I have seen the likes of them all over the internet, and not only does it show that the author is ignorant, it also gives one the impression that not much work has gone into the story at all. And it's not much fun to read. It's like watching a movie with ridiculously blatant continuity errors that no one has bothered to fix, simply because they're so eager to get the movie out so they can get reviews. Trust me, it is well worth the effort.