4、　 有时句子通过but或otherwise等词来表示真实与虚拟的转换，在这种情况下，只有不真实的部分用虚拟式。

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、　 某些条件可以用介词短语without… , but for 等来表示 。

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

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

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

1、　 在if条件句中，如果含有were , should或had等助动词，if可以省去，句子用倒装。

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

4、　 情态动词后还可接不定式的进行式、完成式和被动式。

例如：He can’t be doing his homework now. 他现在不可能在做功课。

He must have attended the meeting last night. 昨晚他一定参加了会议。

(二)虚拟语气

3、　 情态动词的过去式，例如can-could，may-might。但是少数情态动词只有过去式，例如used to表示过去常常。must的过去式往往用had to代替。

2、含有情态动词的句子改为否定句或疑问句不必加助动词，只需要将情态动词置于主语前，或在情态动词后加上not。

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

You needn’t wait for me.

(一)情态动词

1、情态动词在句中没有人称和数的变化。

7．　 在It is time that, I would rather that句式中从句谓语动词用过去式。例如：

It is time we went to bed.

I would rather he came next week.

第二节 实战演练

6．　 在有些句型中如：It is important (necessary、natural、strange等) + that从句中的谓语动词用should + 动词原形，should可以省略。例如：

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