Welcome, Medway Boy.
Here are a few sources for you.
In hypothetical sentences, were is usually used instead of was:
If I were you, I'd learn how to drive.
I wish it were Friday.
It is important to note that was
can also be used (although still considered incorrect by some grammarians), and is, in fact, more common in informal English. Source
form is possible in informal, familar conversation. Source
From the BBC - British Council, a quiz
called How we say things
4. Which of these is not a way to give advice?
a. If I was you, I’d do it again.
b. You should do it again.
c. If you do it again, you’ll be in big trouble. This is a threat.
[Notice that example a. has If I was
, not If I were
. In other words, spoken English can differ from written English.]
About whether you should opt for was or were on an exam:
When the situation described by the if clause is not presupposed to be false, however, that clause must contain an indicative verb.
Does that help?
If Hamlet was really written by Marlowe, as many have argued, then we have underestimated Marlowe’s genius.
If Kevin was out all day, then it makes sense that he couldn’t answer the phone.
Remember, just because the modal verb would
appears in the main clause, this doesn’t mean that the verb in the if
clause must be in the subjunctive if the content of that clause is not presupposed to be false:
If I was (not were) to accept their offer—which I’m still considering—I would have to start the new job on May 2.
He would always call her from the office if he was (not were) going to be late for dinner.Source