If this were mine?

hi, everybody!!
Can anyone help me,why are we sometimes using WERE as a past tense of IS.e.g “if this world were mine”.why don`t we say if this world was mine because the past tense of “is” is “was”

Because it is the subjunctive mood.

Other examples: If I were you, I wouldn’t spend so much money.
It is important that she be there on time.

Hi Elena,Now I think i`m clear.“If i were a girl” can be another example because the mood states a possibillity(not a fact).
Thanks very much,u helped me a lot.