This is an example of the subjunctive mood in English, which is rare. We use it to state something that is contrary to fact. When she says "If I were a boy" she is actually a girl, so she is imagining she were a boy. It's rare and many people would just say "If I was a boy," which is technically wrong but common.

If I were a boy/If she were here/if it were to happen .. If starts off a conditional sentence as in > If you like it / if I give you... But, we use "were" to show that that condition cannot be fulfilled .Its not so much as about something in the past , but rather the impossibility / unlikelihood of it happening. If I were a boy > She cant be a boy If you were here> You cant because you are somewhere else noe . If it were to happen ,.> it didn't happen , you just want to make a point if it happened. If it/she/they /you >> were ( for singular and plural pronouns )

Maria, I'm grateful you for your help. Maria, could you say where looking for this rule.Thanks in advance.

Hello there! It is the rule of conditional sentences. In conditional sentence type 2, instead of using "was" after "I", we use "were". Hopefully it helps! :)