We use "was" with a singular pronoun or noun and "were" with a plural pronoun or noun. An exception to this is the pronoun "you" which always uses "were".
It was a fast car. We were wet.
You were friends. The pupils were happy.
I wasn't hungry. Andy wasn't tall.
The books weren't new. The living room was not neat.
We weren't late. The men were not smart.
Her friends weren't kind. You weren´t at school yesterday.