Positive sentences
I / he / she / it = was ---> I was at a party yesterday.you / we / they = were ---> They were at the party too.
Negations
I / he / she / it = wasn't ---> she wasn't at the party.
you / we / they = weren't --> you weren't at the party either.
Questions
Was I / he / she / it? ---> Was he at the party?
Were you / we / they? ---> Were they at the party?