一、虛擬語氣用於條件狀語從句。
1.表示與現在事實相反的假設時，條件狀語從句中的謂語動詞用“過去式（be的過去式用were）”。而主句中的謂語動詞用“would(should, could, might) 動詞原形” 。例如：If he had time, he would attend the meeting.
2.表示與過去的事實相反的假設時，條件狀語從句的'謂語動詞用“had 過去分詞”，主句中的謂語動詞用“would(should, could, might) have 過去分詞” 例如：
If you had taken my advice, you wouldn’t have failed in the examination.
3、表示與將來事實相反的假設時，條件從句和主句所用的謂語動詞 與“表示與現在事實相反的假設”的謂語動詞相同，或者條件從句中用“were to (should) 動詞原形”，例如：
If it were to rain tomorrow, the meeting would be put off.
4. 當條件狀語從句表示的行爲和主句表示的行爲所發生的時間不一致時，動詞的形式要依據它所表示的時間來調整。例如：
If you had worked hard, you would be very tired.
5.以上句型可以轉換成下列形式：
1）省略if用“were, had, should 主句”。例如：
Were I in school again, I would work harder.
Had you been here earlier, you would have seen him.
但在虛擬條件狀語從句中，省略連詞的倒裝形式的句首不能用動詞的縮略形式，如我們可以說：Were it not for the expense, I would go to Italy.
但不能說：Weren’t it for the expense, I would go to Italy.
2）用介詞短語代替條件狀語從句。例如：
Without air, there would be no living things.
二、虛擬語氣用於賓語從句
1.“wish 賓語從句”表示不能實現的願望，漢語可譯爲
“可惜……”，“……就好了”，“悔不該……”，“但願……”等。
表示現在不能實現的願望，從句的謂語動詞過去式;
表示將來不能實現的願望，用“would(could) 動詞原形”;
表示過去不能實現的願望時，用“had 過去分詞”或“（could） would have 過去分詞”。例如:
I wish it were spring all the year round.
I wish you could go with us.
We wish we had arrived there two hours earlier.
2.虛擬語氣在動詞arrange, command, demand, desire, insist, order, propose, request, require, suggest等後面的賓語從句中用“(should) 動詞原形”。例如：
We suggest that we (should) have a meeting.
We insist that they (should) go with us.
The doctor ordered that she (should) stay in bed for a few days.
He demanded that we (should) start right away.
注意：只有當insist作“堅持（認爲）”，“堅持（應該）”，suggest作“建議”解時，從句的動作發生在謂語之後，賓語從句才用虛擬語氣。當insist作“力言”，“強調”，suggest 作“暗示，表明”解時，賓語從句通常不用虛擬語氣。例如：
Mike insisted that he had never stolen anything.
Madame Curie insisted that there was something in nature that gave out radition.
The expression on his face suggested that he was very angry.
三、虛擬語氣用於表語從句、同位語從句
作advice, idea, order, demand, plan, proposal, suggestion, request等名詞的表語從句和同位語從句，其謂語動詞一般要用虛擬語氣的結構“（should） 動詞原形。”例如：
We all agreed to his suggestion that we (should) go to Beijing for sightseeing.
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