Подскажите . Скоро будет тест по английской грамматике. Когда пишем was , а когда were?

Это формы глагола to be для Past Simple

was только для единственного числа, а were - для множественного (после you всегда следует глагол в форме множественного числа).

(+) I was happy. He was sad. She was pleased. It was great.

(-) I wasn't interested. He/she wasn't surprised. It wasn't funny.

(?) Was I/he/she happy? Was it a great idea?

(+) We were tired. You were angry. They were surprised.

(-) We/you/they weren't tired.

(?) Were we/you/they glad?