As formas do passado to verbo TO BE são WAS (para I, he, she, it) e WERE (para you, we, they).

We were there.
John was here.
It was raining.
I was a student.

Contudo, após IF ("se" no sentido de "no caso de"), deve-se usar WERE para todas as pessoas. É o Caso Subjuntivo, e esse é um erro que até mesmo os nativos de língua inglesa cometem.

If we were there, we'd do it.
If John were here, he'd buy it.
If it were raining, we wouldn't have to do it.
If I were a student, I'd participate in the program.