When I was in High School, I read a rule, that was tricky, and I still remember. The rule is:
If something has happened but we wish it happened the other way by using "if", we should change the was/were in supposedly reverse order with respect to the subject. For example:
If I were a bird. (still remember it from high school)
So I am not a bird, but wish if I was a bird. Here was should be replaced by were.
Now what I understand this rule should apply when something has not happened or something we wish but can't happen. This type of expression can also be conveyed without using "If". But I haven't see this rule used without if. For example Thought it were you (But it isn't)
So my question is Is this rule an if specific rule rule?
Also as a side note: I would also like to know if such rule exists in case of present tense i.e. jumbling am/is/are?