do you put a comma before but

Whether to put a comma before "but" depends on the context and the grammatical structure of the sentence.

In general, when "but" is used as a conjunction to join two independent clauses (i.e., complete sentences that can stand alone), you should use a comma before "but." For example: "She wanted to go to the concert, but she couldn't get tickets."

However, when "but" is used to join two phrases or clauses that are not independent, you typically do not use a comma. For example: "He was tired but happy."

There are also other cases where a comma may or may not be appropriate before "but," such as when "but" is used as a preposition or as part of a parenthetical phrase. In those cases, it's important to consider the specific context and the intended meaning of the sentence.