一、语气的种类

（1）陈述语气 表示动作或状态是现实的，确定的或符合事实的，用于陈述句，疑问句和某些感叹句。如 We are ready. What a fine day it is!

（2）祈使语气 表示说话人的建议，请求，邀请，命令等。如 Open the door, Please.

应注意以下几点：

主语通常是第二人称you，但多不出现，动词用原形，否定用do not 或者don’t 加动词原形（或be） 如 Be careful next time. Don’t smoke here.

有时为了强调，主语也可以出现，而且可以是第三人称，谓语动词不加-s或者-es 如 You be quiet. He stand up.

祈使语气可以用do加强语气 如 Do come to see this Sunday.

在Let’s 的祈使句后，疑问部分通常用shall we；在Let us后，疑问部分用will you 如 Let’s go out for a walk after supper，shall we? /Let us clean our classroom, will you?

祈使句与连词and连用时相当于一个条件句，而and之后则是表示结果。如 Think hard and you will have a good idea.

（3）虚拟语气 表示动作或状态不是客观存在的事实，而是说话人的主观愿望，假设或推测等。如 If I were you, I should study English.

二、虚拟语气在条件从句的用法

条件句有两类，一类是真实条件句；一类是非真实条件句，也就是虚拟条件句。

如果假设的情况是有可能发生的，就是真实条件句，谓语要用陈述语气。如：

 If it doesn’t rain tomorrow, we will go to the park.

如果假设的情况是过去或现在都不存在的，或将来不大可能发生的，则是虚拟条件句。如 If he had seen you yesterday, he would have asked you about it. 在含有虚拟条件句的复合句中，主句和从句的谓语都要用虚拟语气，列表如下：

例如：If he were here, everything would be all right.

If her mother had taken the doctor’s advice, she would/might have got well earlier.

If it were to rain tomorrow, the match would be canceled.

有时候省略if，采用局部倒装语序。把had /should/were 等动词（不包括行为动词）移到从句的句首。例如：

Were it to rain tomorrow, our picnic would be canceled.

Had it not been for the storm, we would have arrived in time.

Should the earth stop running, what would happen?

三、两种特殊条件句

（一）错综时间条件句

有时条件从句的动作和主句动作发生的时间不一致，这时需要根据意思采用表示不同时间的动词形式来进行调整。

If she had taken the doctor’s advice, she might still be alive.

If I were you, I would have accepted their terms.

（二）含蓄条件句

1. 有时候假设的情况并不用条件从句表示出来，而是通过介词短语来表示。

如 Without air(If there were no air), there would be no living things.

But for your help (If it hadn’t been for your help), I couldn’t have done it.

2. 假设的条件通过上下文表现出来。

I would go abroad for further study but that I am poor.

I was ill that day. Otherwise I would have taken part in the party.

3. 表示虚拟语气的主句或从句有时可以省略，但其含义仍可以推知。

（1）省去条件从句 You could have washed your clothes yourself. （你本可以自己洗衣服的。）省去了If you had wanted to。（事实是：你自己没洗衣服，因为你不想洗。）

（2）省去主句（常用以表示愿望）If my grandmother were with me! (如果我的祖母和我在一起多好啊！)事实是：祖母已不在世。