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条件状语从句
条件状语从句就是用以表示“在某种条件下，会……”，引导词主要有 if, unless, as/so long as, on condition that, provided, suppose, supposing 等..
主句用一般将来时，if或unless引导的条件状语从句用一般现在时。
注意：[主将从现原则]在含有条件状语从句的复合句中，表示将来时态，主句是一般将来时态，祈使句或情态动词，从句要用现在时态。
一、if引导的条件状语从句
1） If you ask him, he will help you.
2） If you fail in the exam, you will let him down.
3） If you want to go, please tell me.
4） If you want to know more about it, you can call me.
if引导的条件状语从句既可以将从句放前面也可以将从句放后面
（如果天下雨，我们就不玩了）If it rains, we will stop playing.
转为We will stop playing if it rains.
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。
如：If I were you,I would invite him to the party.
I would have arrived much earlier if I had not been caught in the traffic.
二、其他连词引导的条件状语从句
1.unless = if...not. conj.除非，若不，除非在……的时候
You will fail to arrive there in time unless you start earlier.
如果你不早点动身，你就不能及时赶到那儿。
Unless it rains, the game will be played.除非下雨，比赛将照常进行。
2.on condition(that)...在……条件下，如果 on condition (that)...引导的条件从句是主句事件发生的前提条件或唯一条件。
I can tell you the truth on condition that you promise to keep a secret.
我可以告诉你真相，条件是你答应保密。
You can go swimming on condition (that) you don't go too far from the river bank.
你只有在不远离河岸的条件下才可以下水游泳。
3. supposing conj.如果，假如 supposing引导的条件从句表示一种假设条件。
Supposing it rains, shall we continue the sports meeting?
倘若下雨，我们的运动会还要继续举行吗?
Supposing anything should go wrong, what would you do then?
假如出了什么问题，你准备怎么对付?
4. provided conj.假如，除非，以……为条件 provided (that) +从句表示一种假设条件。
He will sign the contract provided we offer more favorable terms.
如果我们提出更优惠的条件，他就会在合同上签字。
He won't be against us in the meeting provided that we ask for his advice in advance.如果我们提前征求一下他的意见，他就不会在会上反对我们。
5. so/as long as只要
So long as you’re happy, it doesn’t matter what you do.
只要你高兴，你做什么都没有关系。
You may borrow my book as long as you keep it clean.
只要你保持书的清洁，你就可以把我的书借去。
【即学即练】
1.如果我去参加聚会，他们将会很难过。If I ___(go) to the party, they ___(be) upset.
2. 如果我乘出租车去，它将会很贵。If I ___(take) a taxi, it ___(be) too expensive.
3. If she ___(finish) work early, she can go home.
4. If the weather is fine, we ____(go) for a walk.
5. If it ___(rain) next Sunday, we won`t be able to plant trees.
6. Unless you speak to him first, she ___(not speak) to you.
7. If he ___ (call) to you, tell him I will ring back.
8. Have a rest if you ___(be) tired.
9. Don`t come unless I ___(call) you.
10.I don`t know if he ___(come) tomorrow, but he __(come), I`ll let you know.
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