You might say "Hello, everyone" out loud, but I would not start a written sentence with it. It's better to be more formal when writing. Of course, if you have a character in a story who is coming into a place and saying "Hello, everyone!" that is different.
Also, please notice that there should be a comma between "hello" and "everyone."
The correct grammar for this sentence is: When did your friend come?
That sentence is grammatically correct.
"Did you have a chance to speak with him?" is a perfectly good and correct sentence. It does not contain any grammar mistakes.
No, The correct grammar for this sentence would be, "He finished doing his homework."
John's coat is new . The above sentence is correct in grammar and spelling.
no_____If the sentence is You do do that (meaning You are in the habit of doing that) the grammar is perfectly correct and the sentence 'does have correct grammar'.
The correct grammar for this sentence is: "When did you send it?"
That sentence is grammatically correct.
No.
The correct grammar is "Between you and me."
"Roger and I appreciate what you did" is correct grammar
The grammar in the question is correct.
Yes, that sentence is correct grammar.
Yes, "you were correct" is correct grammar. This sentence is in the past tense and conveys that someone was right about something.
The sentence is grammatically correct.
Yes. What is not correct is your putting the article "a" before grammar.
The correct grammar for this sentence is: When did your friend come?