s1、水流年. 1級 2014-01-23 回答

if 的條件從句分為真實條件從句和虛擬條件從句。而真實條件從句適用於主將從現，是假設有可能發生的。而虛擬條件從句是假設不太可能發生的。

IF的虛擬語氣有三種：

內容 從句 主句

與現在事實相反 動詞過去式（be用were） would/could/should/might+動原

與過去事實相反 had+動詞過去分詞 would/could/should/might+have+動分

與將來事實相反 should/were to+動原 would/could/should/might+動原

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。