If 用法汇总
1、主将从现（主句为一般将来时,从句为一般现在时）；
eg:If it rains(从句) ,I will stay at home（主句）.
=I will stay at home if it rains.
2、主句含有情态动词（must、may、can etc.）从句为一般现在时；eg:If you drive too fast ,you may have an accident.
=You may have an accident if you drive too fast
3、主句为祈使句,从句要用一般现在时态；
eg:Don't play computer games if you don't finish your homework. 4、是否=whether
Ask if he is at home. I wonder if she is ill.
5、虚拟语气
eg:If I were a bird ,I would fly in the sky.（事实我不可能是鸟）
时态：可以表示过去，现在和将来的情况，它的基本特点是时态退后.
a.同现在事实相反的假设
从句用一般过去时should( would) +动词原形
If they were here,they would help you.
b.表示于过去事实相反的假设
从句用过去完成时should(would) have+ 过去分词
If she had worked harder,she would have succeeded.
If my lawyer had been here last Saturday,he would have prevented me from going.
c.表示对将来的假想
一般过去时，
were+ to do，从句：主+ should/would/could/might+v原形should+ v原形，
If you succeeded,everything would be all right.
If you were to succeed,everything would be all right.
If you should succeed,everything would be all right.
单项练习（每个必须让学生翻译）
1. If I _____ where he lived, I _____ a note to him.
A. know, would have sent
B. had known, would have sent
C. know, would send
D. knew, would have sent
2.Mary is ill today. If she _____, she _____ absent from school.
A. were not ill; wouldn’t be
B. had been ill; wouldn’t have been
C. had been ill; should have been
D. hadn’t been ill; could be
3. He acted as if he _____ everything in the world.
A. knew
B. knows
C. has known
D. won't know
4. If you had enough money, what _____?
A. will you buy
B. would you buy
C. would you have bought
D. will you have bought
5. If Mr. Young _____ with justice, he _____ such a wrong thing.
A. were treated; wouldn’t do
B. had been treated; wouldn’t have done
C. were treated; wouldn’t have done
D. had been treated; wouldn’t do
6. What _____ if there _____ no light during the day?
A. will happen; was
B. would happen; is
C. happens; were
D. would happen; were
7. The spokesman acted that way as if he _____ anything about the accident but actually he did.
A. doesn’t hear
B. didn’t hear
C. haven’t heard
D. hadn’t heard
8.If it _____ not for the fact that you _____ ill now, I would surely punish you.
A. is; are
B. were; were
C. were; are
D. was; were
9. Many Chinese movie stars now have given up their Chinese nationality. If I were them,I _____ that.
A. shouldn’t do
B. didn’t do
C. hadn’t done
D. won’t do
10. I want to get back by 5 o’clock if _____.
A. likely
B. can
C. possible
D. Probable
11. It seems as if it _____ already summer now but it is only February.
A. is
B. was
C. were
D. had been
