"If I 'were' a rich man." Could it also be "If I 'was' a rich man?" Does it matter?
No, it cannot be "if I was a rich man". It matters in proper writing and if you are taking tests or exams. In informal speech and the writing of ignorant people it is often ignored.
Before, we were using the simple past tense. "None of the passengers was hurt", for example.
However, the phrase "if I were a rich man" uses the past subjunctive. We use the subjunctive to refer to past situations which might (or could) have happened.
The past subjunctive is identical with the past tense except in the case of the verb be, which uses were for all persons: If I were rich
, If he were rich
, If they were rich
The past subjunctive is sometimes called the were subjunctive, since were is the only subjunctive form that is distinct from the indicative past tense. It appears chiefly in if clauses and in a few other constructions expressing hypothetical conditions:
If he were sorry, he'd have apologized by now.
I wish she weren't going away.
She's already acting as if she were going to be promoted.
Suppose she were to resign, what would you do then?