if引导条件状语从句的用法

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.如：
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 have 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.如：
If I were you,I would invite him to the party．如果我是你,我会邀请他参加聚会.
I would have 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.如果你想减肥,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 have 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