Use the imperfect subjunctive tense to talk about things that are extremely unlikely to have happened or happen.
Se eu soubesse o que ela ia fazer... (If I knew what she's going to do...)
Se você falasse que está tudo bem... (If you were to say it's all ok...)
Se eu ficasse mais tempo no Brasil... (If I were to stay in Brazil longer...)
*Also called the Preterit Imperfect or, the Past Subjunctive.