1、虛擬語氣用於條件狀語從句中
（1）表示與現在事實相反的假設，條件狀語從句中的謂語動詞用過去式（be動詞的過去式用were），而主句中的'謂語動詞用would / should/ could / might + 動詞原形。如：
If I were a boy, I would join the army.
If the had time, she should go with you.
（2）表示與過去的事實相反，條件狀語從句中的謂語動詞用過去完成時，主句中的謂語動詞則用would / should / might / could + have +過去分詞。如：
If he had taken my advice, he would have succeeded in the competition.
（3）表示與將來事實相反，條件狀語從句中的謂語動詞一般過去時或should(were to) + 動詞原形，而主句中的謂語動詞則用would / should/
could might + 動詞原形。如；
If it were to rain tomorrow, the football match would be put off.
（4）當條件狀語從句表示的行為和主句表示的行為所發生的時間不一致時，動詞的形式要根據它所表示的時間作相應調整。如：
If they had worked hard, they would be very tired.(從句說的是過去，主句指的是現在）
