Would there ever be a situation where you use "I were", rather than "I was?"
(Not a teacher)
You should look up 'subjunctive mood in English' for further explanation of the differences/rules.
Basically, subjunctive mood in languages indicates uncertain/non-factual statements, or statements that command/request. I'm not sure, but I think the verb 'be' is one of the only verbs which still has a different subjunctive form. It is important to realise that all subjunctive sentences are in the subjunctive mood, but the verb form just doesn't change.
So saying 'If I was' is technically wrong, as 'If...' creates a subjunctive mood.
If the sentence is indicative mood (i.e. factual/certain/not beginning with 'If...'), then it should always be 'I was'.
This is a grammatical rule that is often flouted, and quite rightly so in my opinion. Subjunctive forms have almost entirely disappeared from English apart from the case of 'be'. You will get many authors and formal texts where the subjunctive form isn't used, i.e. "If I was...".
The case where I hear the subjunctive used correctly is when it is someone demanding something; "I ask that you be quiet", where 'be' is subjunctive.