Favorite quotes:

"If we shadows have offended,
Think but this, and all is mended,
That you have but slumbered here
While these visions did appear."-Shakespeare

"He was stupid. If I killed everyone who was stupid I wouldn't have time to sleep." - Alanna to Myles

"His majesty said to come with all deliberate speed."
"That's how we're doing it, deliberately." - Raoul to Messenger

"You do realize we should all be put in a room with lots of muscular men to keep us from harming ourselves?" - Neal

"When people say a knight's job is all glory, I laugh, and laugh, and laugh. Often, I can stop laughing before they edge away and start talking about soothing drinks" - Sir Lord Raoul of Goldenlake and Mallorie's Peak

"Silence, insubordinate curs! Do not sully my new squire's ears with your profane tales!"
"Even if they're true?"- Raoul and Dom

A few sentences I found hilarious:

“Victor,” Eros said, revealing himself finally in the dark, and raising his eyebrows. “You have a tail. I think that Love is quite possibly more believable than that…” -from the fanfic Stolen Away by Proud Titania

"Where's Voldemort when you need him," he mumbled, "Probably plotted to get here but chickened out after seeing Amelia was here. Just like him to make me suffer alone. Stupid prat."- from the fanfic Insidious Inquisitor by Yunaine

As he(Voldemort) reached for the doorknob, he turned around and added, “Oh, and try to do something with your hair, it looks terrible.”
It was a good thing that Snape was a sufficiently accomplished Occlumens, otherwise Voldemort would likely have killed him since his next thought was, “You’re just jealous because I have hair.”
The Flying Lesson by Hypatia

Some common mistakes I find and ways to fix them and/or identify them (they're not in any particular order):

1. 'I' always comes before a verb, 'me' comes after the verb.

Ex.) Sally and I went to the park.
They told my brother and me about the small puppy in the shop window.

2. Loose is often mistaken for lose.
Loose- adj. not tight, not attached or confined; verb form- loosen, meaning to make less tight; loosing- to make less tight; loosening- to become less tight
Lose- v. to be deprived of or to rid oneself of; past tense- lost; past participle- have/had lost; present tense- losing; noun- loss

Ex.) I don't want to lose you.
The clothes were a little loose on me.

3. The correct usage of a semicolon/commaconjunction is very commonly misused.
A semicolon is used to connect two complete sentences(nounverb) that are similar. A commaconjunction(and, but, or) is used in the same way.

Ex.) We turned on the fan; it was hot.
Sarah likes to buy shoes, and she went shopping today.

4. A colon introduces a list, but there has to be a complete sentence first.

Ex.) There are all types of people in the world: black people, white people, Asian people, fanatical people, boring people, etc.

5. The words 'too' and 'to' are not interchangeable and neither are 'than' and 'then.'
Too- used when explaining to what extent or to replace the word 'also'(when replacing the word also, there are commas around it)

Ex.) I, too, have a secret.
There are too many people here.

To- usually used before a verb in the infinitive form(to dance, to talk, to ride, etc.), before a destination, and when 'too' is not in use.

Ex.) I would love to go dancing.
We went to the park.

Than- Used when comparing things(more than, less than, other than, etc.)

Ex.) She is a better artist than I am.
There are less circles than squares.

Then- used when 'than' can't be applied correctly(it's the best explanation I came up with, sorry)

Ex.) He then came up with a most brilliant idea.
Then, the whale splashed water over the spectators.

6. Apostrophes are not something to be thrown around, and although sometimes people use them to make something plural, it's not supposed to be used that way.

An apostrophe is used to show possessiveness(it's commonly 's for singular, s' for plural), quotes inside a quote(Sarah said, "I'd love to read W.B. Yeats' 'The Stolen Child' again."), and to make contractions(can't, won't, etc.)
Commonly, when making something plural, just add an s, no apostrophe is needed unless it fits one of the criteria above.

Ex.) James' head was getting big again.
Fred's dog died.
I can't work tomorrow.
The Nguyens are coming over tomorrow.

7. Correct usage if its, it's, your, you're, their, they're, and there are highly encouraged.
Its is possessive, it's means it is.
Your is possessive, you're means you are.
Their is possessive, they're means they are, and there shows where.

Ex.) The dog hurt its paw.
It's one of the best books I've read.
Your book is on the bed.
You're the best friend I've ever had.
Their dog ran away.
They're coming for the boy.
There is the car.

8. After 'would,' 'could' or 'should' is always have. People tend to use 'of,' which doesn't make sense if you think about it.

Ex.) I should have done my homework today.

9. Aside from the quotations marks, there is always punctuation separating the dialogue from the speaker. For the most part, that punctuation is a comma, but if it is an exclamation or a question, punctuate accordingly.

Ex.) Sarah said, "I'll be home late today."
Billy asked, "When is the concert?"
"Look out!" Jen exclaimed/shouted.

10. When addressing someone(usually in dialogue), the name of the person is always surrounded by commas. This is not true if the name of a person is mentioned in passing or if he/she is doing something.

Ex.) "Harry, Jen went to go play in the sand," said Eric to his friend.

11. Knowing when to use 'who' or 'whom' in a sentence is tricky.
'Who' is used when it's the subject of a sentence. If you can replace it with 'he' or 'she,' then use 'who.'
'Whom' is used when it's the object of a verb or preposition. If it can be replaced with 'him' or 'her,' use 'whom'(rephrase the sentence if necessary when using this trick).

Ex.) Who is coming today?(Is she coming today?)
Whom are we going to see today?(Are we going to see him today?)

12. 'Good' is misused frequently in the English language.
'Good' describes a noun.
'Well' describes a verb.

Ex.) You do good things well('good' is describing 'things,' 'well' is describing the act of doing).
That was a good game.
I am well.

13. The usage of 'has' and 'have' can be tricky and people often mistakenly substitute one for the other.
'Has' almost always follows a singular subject (he, she, it, etc.) to indicate possession or as part of a past participle spoken in present tense. The exception to this rule are the pronouns 'you' and 'I'. 'Has' does not follow either of those two words.
'Have' usually follows a plural subject (they, we, etc.) or the pronouns 'you' and 'I' to indicate possession or as part of a past participle spoken in present tense.
'Had' indicates past tense, and it is also used to indicate past when one is already talking about the past. (Basically, if you're recounting a story, you're already telling it in the past tense, right? If you wanted to say that something already happened during the story, then you use 'had'.) 'Had' can follow both a singular or plural subject.

Ex.) He has a dog.
She has decided to ride her bike home.
He has already eaten.
They have a large house.
I have all my stuff packed and ready to go.
When Jenny got into the car, she realized she had forgotten her keys in the house.
They had a huge fight earlier.

14. 'Accept' and 'except' are two words which are often mistaken for each other.
Accept- v. meaning to take, receive, or consent to something
Except- a preposition that is used in place of 'unless', 'but' 'other than', 'apart from', 'only'; also a verb which means 'to exclude'

Ex.) I accepted his apology because I felt sorry for hi,m.
I love everything except the lamp.