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When do we use a comma before "but"? Is it optional or obligatory to use it before "but"?


This question is too general because but has different functions. It is usually a conjunction but it can also be a preposition, as in this sentence:
I saw everyone but John at the party.

And as a conjunction, but can join words, phrases, or clauses.

I like apples but not bananas.
Jack washed the dishes but left them for Jill to dry.

When but is used to join independent clauses (sentences), we usually use a comma at the end of the first clause. However, if the two clauses are short and closely related, the comma is optional. In modern usage, it is preferred to omit the comma in this case.

Hilary grabbed her books and rushed to the door, but she caught her foot on the carpet and tripped. (long clauses; comma is needed)
Our team is strong but theirs is weak. (short, closely related clauses; comma optional but preferred omitted)