The first rule we remember is that for the first and third singular person we should use was; and then we should use were for the others.

So when writing or talking in past tense, the sentence should be like this”

I was in the bathroom when the earthquake began. 

Last year, you were a high school student.

Erica was sad this morning.

However, this rule does not apply when we are using it in an if clause for an unreal condition. Unreal condition means that the thing that we are talking about in the sentence is something that has not happened yet or very unlikely or impossible to happen. In this case, whatever the subject is, we should use were.

Example:

If I were a bird, I would fly to Paris every morning for breakfast.

Thomas would be a businessman right now if he were not so stubborn.