It's if I were for hypothetical in the present or future and if I was when talking about something presumed true in the PAST. "IF" then means something likened to "since".
If I were class president, I would represent our class very well for the next four years.
If I was at the party last night, I don't remember.
It's an old, residual rule from the days of yore when English verbs conjugated differently for person and singular/plural in both the past and present tense indicative and subjunctive. While I may not have enough knowledge on Old and Middle English, I can show you the conjugation for to do for the 2nd person singular form of "thou" in Early Modern English:
present indicative - thou dost
present subjunctive - thou do
past indicative - thou didst
past subjunctive - thou did
It's why it should be "if I be" for things possible and one could still say it. We see "if truth be told" and "whether it be" and others, all residual subjunctives from the days of Chaucer and even Shakespeare wherein it was already disappearing. In Modern English, the past tense is uniform for every person (I, you, he, we, you, they did) except for "to be" (I, he was, but you, we, you, they were), but it wasn't always that way.
Anyway, if I were you, I would learn it because it usually separates the intelligent from ignoramuses. It's correct English even if it be a little old.