What is the rule for wasn't and weren't?

The rule for using "wasn't" and "weren't" is based on the subject of the sentence. "Wasn't" is the contraction of "was not" and is used with singular subjects (I, he, she, it) in the past tense. In contrast, "weren't" is the contraction of "were not" and is used with plural subjects (you, we, they) and the singular "you" in the past tense.