Here's why:
* Independent clauses are complete sentences that can stand alone.
* Conjunctions join words, phrases, or clauses.
Example:
* Incorrect: I went to the store, but I forgot my wallet.
* Correct: I went to the store, but I forgot my wallet.
Note: There are some exceptions to this rule.
* If "but" is used to introduce a dependent clause, you don't need a comma before it.
* If "but" is used as a preposition (meaning "except" or "without"), you don't need a comma before it.
Example:
* Dependent clause: I went to the store, but I didn't buy anything. (The clause "but I didn't buy anything" can't stand alone).
* Preposition: Everyone went to the party but me. (The word "but" is functioning as a preposition, meaning "except").
If you're unsure about whether to use a comma before "but", it's always best to err on the side of caution and use one.