if 的條件從句分為真實(shí)條件從句和虛擬條件從句.而真實(shí)條件從句適用于主將從現(xiàn),是假設(shè)有可能發(fā)生的.而虛擬條件從句是假設(shè)不太可能發(fā)生的.
IF的虛擬語氣有三種：
內(nèi)容 從句 主句
與現(xiàn)在事實(shí)相反 動(dòng)詞過去式（be用were） would/could/should/might+動(dòng)原
與過去事實(shí)相反 had+動(dòng)詞過去分詞 would/could/should/might+have+動(dòng)分
與將來事實(shí)相反 should/were to+動(dòng)原 would/could/should/might+動(dòng)原
下面三句依次是上面三種情況的例句：
If i were you ,I would/could/should/might do that.
If i had know that yesterday,I would/could/should/might have told you .
If it should rain ,the crops would/could/might be saved.