“Before we went to the festival_ we stopped at the bank.”
The sentence begins with a prepositional phrase, which contains 6 words; therefore, a comma is required.
The rule is that a comma is necessary when the sentence begins with a prepositional phrase of at least 4 words. Therefore, the sentence
“While it rained we played Clue.”
does not require a comma.
Reblog if you already knew this Grammar Tidbit, Grammarphiles!