It depends on the sentence.
That depends on how you are using the word. It isn't always necessary, but if the word "and" is joining two independent clauses (independent clauses are basically sentences that can stand on their own), then you definitely need a comma before the word and, or any coordinating conjunction. Examples: Apples and oranges may be different, but they are both delicious. I like to eat apples, and I love orange juice. In the first example, a comma is not necessary before the word and because it is just joining two words together. In the second, the comma is necessary because the word and is joining two sentences together.
A comma before "which" depends on whether it introduces a nonessential clause. If the information following "which" is necessary for the sentence's meaning, no comma is needed. However, if the clause is nonessential, a comma before "which" is appropriate.
It is not necessary to use a comma immediately following the word "hence." However, if the phrase following "hence" is a separate clause, then a comma may be used to separate the clauses.
A comma should typically come before the word "but" when it is used to connect two independent clauses. If "but" is used to join two elements within a single clause, then a comma is usually not necessary.
You can put a comma before or after just about any word if the sentence structure requires it. If the sentence structure does not require it, it may be permissable to use a comma to assist in clarity and avoid confusion. Otherwise, don't use a comma. Have I confused you yet? Using a comma does not depend on the word, it depends on the structure of the sentence.
Typically, there is no need for a comma before the word "as." However, using one is not technically wrong, just superfluous.
No, a comma does not go before the word 'in'.
No, a comma is not necessary before "in turn" in most cases. "In turn" is typically used to show a sequence of events or actions and does not require a comma when used in this way.
A comma should be used before the word "but" when it is connecting two independent clauses in a sentence. If "but" is joining two words or phrases within a single clause, then a comma is not necessary.
It depends on the context. It can be correct to put a comma before "then" or after "then", or not to have a comma either before or after "then". There is no word or phrase in English that requires a comma. Examples: Turn left at the light, then take the next right, and then take the second right after that. Let's eat dinner now, and then, if you'd like, you can play video games.
A comma is not necessary before "then" in most cases. However, if "then" is used to indicate a transition or contrast, a comma may be appropriate.
Not necessarily. There is no word in English that requires a comma before or after it.