If + were or if + was

That's a question many ESL students ask. The answer is: usually either were or was can be used, the former being more likely in formal English

• If she was/were offered the job she'd take it.
• If Tom was/were here he'd know what to do."

But were is a little more usual than was in the advice form If I were/ was you I would/should . . . :

1. "If I were you I would wait a bit," he said. 

Were is also more usual in the infinitive construction:

• If Peter were/was to apply for the post he 'd get it.

However: Were, not was, is used when the auxiliary is placed first:

• "Were I Tom I would refuse."

When if means "since" was cannot be replaced by were, were can replace was after if only and wish.