定义：常见的if条件状语从句表示在某条件下，某事很可能发生，条件是可能存在的，主句中某种情况发生的概率也是很高的。
if you ask him，he will help you．
如果你请他帮忙，他会帮你的。
if you fail in the exam，you will let him down．
如果你考试不及格，你会让他失望的。
if you h**e finished the homework， you can go home．
如果你作业做完了就可以回家了。
另外，if从句还表示不可实现的条件或根本不可能存在的条件，也就是一种虚拟的条件或假设。从句多用一般过去时或过去完成时，表示对现在或过去的一种假设。
i were you，i would invite him to the party．
如果我是你，我会邀请他参加聚会。
would h**e arrived much earlier if i had not been caught in the traffic．
要不是交通堵塞，我本会来得早一些。
另外你还要注意if 条件句的时态搭配。
1．if从句用一般现在时，主句用一般将来时
if he runs he’ll get there in time.
如果他用跑的，他就会及时赶到那儿。
the cat will scratch you if you pull her tail.
如果你拉猫的尾巴，它就会抓你。
2．if从句用一般现在时，主句用may/might/can
if the fog gets thicker the plane may/might be diverted.
如果雾在大一些，飞机可能就会改在别的机场降落。
if it stops snowing we can go out.
如果雪停了，我们就可以出去。
3．if从句用一般现在时，主句用must/should
if you want to lose weight you must/should eat less bread.
如果你想**，你必须少吃面包。
4．if从句用一般现在时，主句用一般现在时。
if you heat ice it turns to water. (也可用will turn)
如果把冰加热，它就会化成水。
5．if从句用现在进行时，主句用一般将来时。
if you are looking for peter you’ll find him upstairs.
如果你是在找彼得，上楼就会找到他。
6．if从句用现在完成时，主句用一般将来时。
if you h**e finished dinner i’ll ask the waiter for the bill.
如果你吃完了，我就叫服务生来算账。
注意 ：学习" if " 引导的条件状语从句的用法，现在总结一下：
if 条件句不一般，几个要点记心间；
条件句，放在前，逗号要放句中间。
条件句表可能，主句多用将来时；
条件句表事实，主句常用现在时。
在 if 引导的条件状语从句中，如果从句谈论的是一个有可能发生的事实及其产生的相关的结果，主句用一般将来时态，从句用一般现在时态。如：
we can walk there if we can't find a bus .
if it rains tomorrow ,we will not go to the zoo.
what will you do if you find a panda in danger.
如果 if 条件句谈论的是重**生和预示要发生的情景和事件，则主从句大多用一般现在时态。
if bears are in danger ,they attack people.
在 if 引导的条件状语从句中，if 和条件句位置灵活，可直接放在主句后面，若 if 条件句放句首， 从句后面要加逗号，和主句隔开。还要注意前后时态一致原则。
经典练习】一。单项选择。
）1. if you __to the party, you’ll h**e a great time.
a. will gob. went
c. god. going
）2. it will be a long time __peter __his work.
has finished b. after, finishes
c. when, will finish d. before, finishes.
）3. what will father __us from japan?
a. takeb. bring
c. carryd. make
）4. weifang is famous __kites.
a. forb. to
c. ond. with
）5． i __her the answer if she __me.
a. can tell, will askb. will tell, will ask
c. would tell, askd. will tell, asks
）6. –what are you going to do tomorrow?
--we’ll go to the library tomorrow if it __
a. isn’t rainb. rain
c. won’t raind. doesn’t rain
）7.—do you know when he will come back tomorrow?
sorry, i don’t know. when he __back, i’ll tell you.
a. comesb. will come
c. comed. may come
）8. what will you do if you __to the old folk’s home visit?
a. gob. went
will go
）9. if i eat __food, i’ll be very fat.
a. too manyb. many too
c. too muchd. much too
）10. i’ll give the book to him if he __here next sunday.
a. will comeb. comes
c. is comingd. came
）11. there __an english film in our school tomorrow.
a. is going to h**eb. will h**e
c. is going to bed. has
）12. could you tell us where __
a. will the next olympic games held
b. the next olympic games will be held
c. would the next olympic games be held
d. the next olympic games would be held
二．在正确的答案下划一条线。
1. daina ( will go/go/goes) to europe if she (will pass/pass/passes) the exams .
2. the graduates (will teach/teach/teaches) in the poor village if the ministry of education (will agree/ agree/agrees) soon.
3. if there (will be/ are/is) a car accident, they (will call/call/calls) 110 for help at once.
4. if it (won’t/don’t/doesn’t/ isn’t) rainy, we (will take/ take/takes) walk outside.
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