You have to learn the rules before you can break them. At least that's what our English teachers told us when we cited Dickens as a defense for our use of run-on sentences.
It's true that not all grammar violations are created equally. Some indicate a blatant disregard for, or ignorance about, what's commonly accepted. These are the result of laziness, cluelessness, or lack-of-a-copy-editor-ness. And they're not okay.
But other rule-bending choices demonstrate a command of language, and an ability to wield it (e.g. this sentence, which began with a sensical and aesthetically-pleasing conjunction). These tweaks can enhance the theme or mood of a story, rather than distracting readers or inducing face-palms.
Here are 7 authors who beautifully broke the language and grammar rules your high school English teachers taught you:
Frequent user: Jane Austen
Example: "She owned that, considering every thing, she was not absolutely without inclination for the party." from Emma
Why it's okay: Jane Austen did not neglect to use double negatives in her writing. Often, as in the example above, she used redundancy to denote a character's snootiness. To show that the grand dame of Highbury was hesitant to act excited about a dinner party, Austen chooses to express her support convolutedly. The effect, more often than not, was to note the humor in the pretentiousness of high society.
Frequent user: Charles Dickens
Example: "It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair..." from A Tale of Two Cities
Why it's okay: Dickens's long sentences are not the result of being paid by the word, contrary to popular belief (in fact, his contracts for books such as Bleak House were based on sales, not word count). His long-windedness served a purpose: It was often meant to satirize the rambling speech of those affiliated with the institutions that he often criticized.
Frequent user: E.E. Cummings
Example: "i sing of Olaf glad and big/whose warmest heart recoiled at war" from "i sing of Olaf glad and big"
Why it's okay: In general poets take more liberties with grammatical rules than novelists, who are expected to communicate using paragraphs and sentences rather than more contracted observations. Cummings was especially experimental, and often used a lowercase "i" (also seen in the first line of one of his well-known love poems: "somewhere i have never travelled, gladly beyond"). In "i sing of Olaf glad and big," the lowercase "i" is contrasted by the uppercase "Olaf," denoting the unimportance of the teller of the poem relative to the poem's subject. His name is also sometimes written with unconventional orthography (e e cummings).
Starting a sentence with a conjunction
Frequent user: William Faulkner
Example: "He had never seen a home, so there was nothing for him to say about it. And he was not old enough to talk and say nothing at the same time." from Light in August
Why it's okay: Faulker, writer of five-word long chapters, bent all sorts of grammar rules. He, like Cummings, had an orthography of his own, and took liberties with capitalization. He also began sentences with conjunctions, seeming to adhere to the rule that a sentence should simply be a complete thought. The above quote contains two sentences, each with disparate but connected thoughts, so separating it into two sentences is phonetically pleasing.
Frequent user: H. L. Mencken
Example: "Ticket-sellers in the subway, breathing sweat in its gaseous form.... Farmers plowing sterile fields behind sad meditative horses, both suffering from the bites of insects....Grocery-clerks trying to make assignations with soapy servant girls...." from "Diligence," A Mencken Chrestomathy
Why it's okay: Shakespeare, Dickens, and just about every other classic author has used some form of an incomplete sentence, including the above-mentioned sentences beginning with a conjunction. There are plenty of reasons for doing this, such as creating a sense of alarm with onomatopoeias, or a sense of anxiety with a paragraph full of choppy fragments. Mencken's purpose was the latter.
Ending a sentence (or independent clause) with a preposition
Frequent user: William Shakespeare
Example: "We are such stuff as dreams are made on; and our little life is rounded with a sleep." from The Tempest
Why it's okay: This "rule" is mostly considered a myth by most grammarians. Grammar Girl uses the example of, "what did you step on?" It'd sound ridiculous and too formal to say, "on what did you step?" out loud. If a sentence or independent clause is more aesthetically pleasing when it's arranged with the preposition at the end, then so be it. This quote would sound clunky if it were restructured to read, "Dreams are made on such stuff as we are."
The subjunctive/passive voice
Frequent user:Sir Arthur Conan Doyle
Example: "'No, your boy had been observed, and that gave me a guide where to look." from The Hound of the Baskervilles
Why it's okay: The passive voice can make for unclear writing, but sometimes, unclear writing is useful. When the subject of a sentence should be a mystery to the reader, the passive voice is an excellent way to achieve that effect. This trick is used by politicians looking to shirk blame, and is a great tool for thriller writers. "The girl had been murdered" is more gut-wrenching than "someone murdered the girl," because it puts "the girl" at the forefront of the sentence without revealing who the murderer is.