1.虚拟语气用在由as if, as though 引导的方式或让步状语从句中，一般情况下，如果主谓语动词为现在时，从句谓语多用过去时；如果主句谓语为过去时，从句谓语多用过去完成时。虚拟语气用在 even if, even though, whatever引导的状语从句中，从句谓语可参照由if引导的虚拟条件从句的谓语，一般用过去时或过去完成时。例如：
This old machine runs as if it were a new one.
The old worker works as though he were a young man.
Even though she had been here very soon, she would not catch the train.
Example:
He has just arrived, but he talks as if he ______ all about that.
a. know b. knows c. known d. knew
I can't stand him. He always talks as though he _____ everything.
a. knew b. knows c. has known d. had known
______ I used a lot of examples I still could not get my idea across.
a. Since b. But that c. Now that d. Even though
2.虚拟语气用在so that, lest （唯恐）, in case （万一）, unless, in order that（为了，以便）, that（为了，以至于）, for fear that （生怕，以免）, on condition that （如果）等引导的目的，条件状语从句中，从句谓语 may 或 might+原形动词，也有用 should, can, could+原形动词构成的。例如：
The teacher explained the law once more in order that we might understand it better.
Some people eat so that they may live. Others seem to live in order that they may eat.
一些人吃是为了生存，而另一些人似乎是活着就是为了吃。虚拟语气用在其它状语从句中
I left him a note with my address on for fear that he should not know where to see me.
The heroes gave their lives that we might live a happy life.
为了我们过上幸福生活，英雄们献出了自己的生命。
Example:
In case I ______, I would try again.
a. will fail b. would miss c. should fail d. shall miss
You should exercise vigilance _____ you should be killed.
a. so that b. lest c. therefore d. that
3.虚拟语气用在由 “whether” 引导的让步状语从句中，从句谓语用原形动词。例如：
All engines work on this principle, whether they be large or small.
所有发动机都按这一原理工作，无论它们是大还是小。
We must finish the work before we go home, whether it be early or late.
无论时间早晚，我们必须在回家前干完工作。
All matter, whether it be gas, liquid or solid, is made up of atoms.
所有物质，不论是气体，液体，或固体，都是由原子构成。
在此句型中，可省去 whether ，而将be放在主语前。例如：
Be it so, we must continue to do the test.
We must do our best to fulfil the task, be it ever so hard.
Example:
______ I'll marry him all the same.
a. Were he rich or poor b. Be he rich or poor
c. Whether rich or poor d. Being rich or poor
练习题：
1.______, I must do another experiment.
a. It is ever so late
b. Ever so late it
c. Be it ever so late
d. Whether be it ever so late
2.The young driver looked over the engine carefully lest it ______ on the way.
a. goes wrong b. would go wrong
c. went wrong d. should go wrong
3.Machines are often oiled so that they ______ well.
a. ran b. runs c. should run d. running
4.Even if he ______ very busy, he would have kept on learning English.
a. had been b. was c. were d. had being
5.Supposing the weather ______ bad, where would you go?
a. is b. will be c. was d. were
6. ______, he could not succeed.
a. However, he tried that
b. He tried however hard
c. However he could try
d. However hard he tried
7. Even though the enemy ______ wings, they couldn't escape from our encirclement.
a. had b. had had c. have d. has
8.She described the accident as if she ______ it herself.
a. saw b. has seen c. had seen d. sees
9.He came to meet us at the station for fear that we ______ the wrong way.
a. take b. should take c. took d. had taken
10.The car looked very dirty, as though it ______ hundreds of miles.
a. run b. runs c. has run d. had run
11.The teacher spoke slowly in order that we ______ no difficulty in following him.
a. should have b. had
c. will have d. didn't have
12.All magnets behave the same, ______ they large or small.
a. how b. are c. be d. were
13.At the beginning he didn't want to speak English lest he _____ mistakes.
a. will make b. should make
c. would make d. can
14. I remember the whole thing as if it ______ yesterday.
a. happen b. happens c. happened d. happening
15.She stayed at home for a few days so that she _____ her mother.
a. takes care of b. took care of
c. might took care of d. might take care of
16.She took up the meter with care for fear that it _____ .
a. is damaged b. to be damaged
c. should be damaged d. was damaged
17. In many cases the heating effect is neglected as if there ______ no flow of current along the wire.
a. are b. were c. is d. being
18.I'll put the money on the table just in case you ______ it.
a. might need b. would need
c. needed d. may have needed
19.Providing you ______ the opportunity to go abroad, which country would you want to visit first?
a. have b. will have c. had d. have had
20.The business of each day, ______ selling goods or shipping them, went quite smoothly.
a. it being b. was it c. it was d. be it
