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4、　 有時句子通過but或otherwise等詞來表示真實與虛擬的轉換，在這種情況下，只有不真實的部分用虛擬式。

例如：I was ill yesterday；otherwise I would have played football with you .

But for the heavy rain , we would have gone to the concert .

3、　 某些條件可以用介詞短語without… , but for 等來表示 。

例如：Without your help , I could not have succeeded in passing the exam .

2、　 should與would都可以用于if條件句的主句中，但should一般只用于第一人稱，would用于所有人稱。

例如：If you were here , I would tell you the news .

1、　 在if條件句中，如果含有were , should或had等助動詞，if可以省去，句子用倒裝。

例如：Were I you , I would buy this reference book.

4、　 情態動詞后還可接不定式的進行式、完成式和被動式。

例如：He can’t be doing his homework now. 他現在不可能在做功課。

He must have attended the meeting last night. 昨晚他一定參加了會議。

(二)虛擬語氣

3、　 情態動詞的過去式，例如can-could，may-might。但是少數情態動詞只有過去式，例如used to表示過去常常。must的過去式往往用had to代替。

2、含有情態動詞的句子改為否定句或疑問句不必加助動詞，只需要將情態動詞置于主語前，或在情態動詞后加上not。

例如：Must I write a letter to cheer him up?

You needn’t wait for me.

(一)情態動詞

1、情態動詞在句中沒有人稱和數的變化。

7．　 在It is time that, I would rather that句式中從句謂語動詞用過去式。例如：

It is time we went to bed.

I would rather he came next week.

第二節 實戰演練

6．　 在有些句型中如：It is important (necessary、natural、strange等) + that從句中的謂語動詞用should + 動詞原形，should可以省略。例如：

It is necessary that the doctor (should) be sent there at once.