Was e were - o passado do verbo to be

O verbo to be é aquele nosso velho amigo que significa ser ou estar. Geralmente é um dos primeiros pontos de gramática que se aprende no inglês. Mais adiante nos estudos ele aparece no passado, com o was e were. As regras aplicadas ao was/were são as mesmas aplicadas a am/is/are, que são as formas do to be no presente.

Pra dar uma revisada no to be no presente, clica aqui

Na frase afirmativa, é só lembrar da conjugação:

I was              \
You were         \
He was             \
She was             \ at home yesterday.
It was                /
We were          /
You were       /
They were   /

Na frase negativa, é só adicionar um not depois da forma do to be, então temos wasn't (was not) e weren't (were not)

I wasn't              \
You weren't         \
He wasn't             \
She wasn't             \ at home yesterday.
It wasn't                /
We weren't          /
You weren't       /
They weren't   /

Nas perguntas, ocorre a inversão da posição entre o sujeito e o verbo:

Was I          \
Were you     \
Was he          \
Was she          \  at home yesterday?
Was it              /
Were we        /
Were you     /
Were they  /

As perguntas que começam com was/were exigem short answers (yes/no):
Yes, I was./No, I wasn't.
Yes, you were./No, you weren't.
Yes, he was./No, he wasn't.
Yes, she was./No, she wasn't.
Yes, it was./No it wasn't.
Yes, we were./No, we weren't
Yes, they were./No they weren't.

Se for uma information question (aquelas que começam com um wh-), o wh- sempre aparece no início:

Where were you yesterday?
What was your favorite food when you were a child?
Why were you so happy yesterday?

...

Então dá uma olhada nestes testes aqui:
Teste 1
Teste 2