4、　 有時(shí)句子通過(guò)but或otherwise等詞來(lái)表示真實(shí)與虛擬的轉(zhuǎn)換，在這種情況下，只有不真實(shí)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(yǔ)without… , but for 等來(lái)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(chēng)，would用于所有人稱(chēng)。

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(dòng)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(tài)動(dòng)詞后還可接不定式的進(jìn)行式、完成式和被動(dòng)式。

例如：He can’t be doing his homework now. 他現(xiàn)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(huì)議。

(二)虛擬語(yǔ)氣

3、　 情態(tài)動(dòng)詞的過(guò)去式，例如can-could，may-might。但是少數(shù)情態(tài)動(dòng)詞只有過(guò)去式，例如used to表示過(guò)去常常。must的過(guò)去式往往用had to代替。

2、含有情態(tài)動(dòng)詞的句子改為否定句或疑問(wèn)句不必加助動(dòng)詞，只需要將情態(tài)動(dòng)詞置于主語(yǔ)前，或在情態(tài)動(dòng)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(tài)動(dòng)詞

1、情態(tài)動(dòng)詞在句中沒(méi)有人稱(chēng)和數(shù)的變化。

7．　 在It is time that, I would rather that句式中從句謂語(yǔ)動(dòng)詞用過(guò)去式。例如：

It is time we went to bed.

I would rather he came next week.

第二節(jié) 實(shí)戰(zhàn)演練

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

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