The imperfect subjunctive tense is used to talk about things that are extremely unlikely to have happened or happen. It’s most often used together with the conditional tense:
Se eu soubesse tudo seria sábio. (If I knew everything I'd be wise.)
Se fosse ele eu te contaria. (If it were he I would tell you.)
Se eu tivesse tempo estudaria mais. (If I have time I'd study more.)