if I was / if I were
A past fact versus a hypothetical.
Use if I were for a hypothetical or a counterfactual: If I were chair, I would wait. Use if I was for a past fact that may have been true: If I was late, I am sorry. Do not use if I was for a present impossibility.
Examples
Use If I were richer, I would rest.
Use If I was rude, I apologize.
Avoid If I was richer, I would rest.