Use the imperfect subjunctive tense to talk about things that are extremely unlikely to have happened or happen.
*Also called the Preterit Imperfect or, the Past Subjunctive.
Se eu soubesse... (If I knew / were to know...)
Se você falasse... (If you said / were to say...)
Se você tivessel... (If you had / were to have...)
It’s most often used together with the conditional:
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.