Past Simple of the verb "To be"
The simple past of the verb "To be" (to be) is formed by changing the forms "am" and "is" for "was" and how "are" with "were".

Moreover, to form the negative and interrogative form is the same as in the present simple.

Let's see the ways affirmative negative interrogative verb "to be" in the past:
Afirmativa:

I was: yo era o yo estaba.
You were: tú eras o tú estabas.
He was: él era o él estaba.
She was: ella era o ella estaba.
It was: eso era o eso estaba.
We were: nosotros eramos o nosotros estábamos.
You were: vosotros erais o vosotros estabais.
They were: ellos eran o ellos estaban.

Negativa:

.
It was not: eso no era o eso no estaba.
We were not: nosotros no eramos o nosotros no estábamos.
You were not: vosotros no erais o vosotros no estabais.
They were not: ellos no eran o ellos no estaban.

Interrogativa:

Was I?: ¿era yo? o ¿estaba yo?
Were you?: ¿eras tú? o ¿estabas tú?
Was he?: ¿era él? o ¿estaba él?
Was she?: ¿era ella? o ¿estaba ella?