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问题描述
现在完成时可以用在条件或时间状语从句中, 表示将来某个时刻之前已经完成的动作。
e.g. I’ll go home as soon as I have finished my homework.
Please lend me that book if you have finished reading it. 怎么理解呢？可不可以把后面的完成时的从句理解成将来完成时呢？那样的话就是从现在看将来某个时间这个动作已经完成呢？在以上2个例子中，每个句子中的两个动作都是假设的，都没有发生对吧？另外，现在完成时是说目前已经完成的动作，怎么还可以表示将来某个时刻之前已经完成的动作呢？是不是把它理解成从现在看将来某个时刻之前已经完成的动作？谢谢。
老师答疑
刘老师：
I’ll go home as soon as I have finished my homework.
Please lend me that book if you have finished reading it.
【答】你的问题，从本质上说就是：在“时间状语从句”或者“条件状语从句”中，我们用现在完成时代替将来完成时,表示将来业已完成的动作或状态。这跟“主将从现”是一个道理。当然，将来的动作现在肯定还没有发生，这里的现在完成时也是将来发生。从句中，一般现在时代替一般将来时；同理，现在完成时代替一般将来完成时。
例如：
1. 时间状语从句：until, as soon as, when
Stay in the house until the danger has passed. 躲在家里，直到危险过后再出来。
I'm very busy just now; wait until things have eased up a little. 我现在非常忙，等事情缓一缓再说。
Don't drink the water until it has been cooled. 直到水冷了之后再喝。
We can't move in until the electricity has been laid on. 我们得等电源接通后才能搬进去。
I'll go to my granny's as soon as I have finished my homework. 我一完成作业就去奶奶家。
As soon as he has regained his memory, the problem will be solved. 只要他一恢复记忆，问题就可以解决了。
When you have formed a plan, you must carry it out. 计划订出之后，你必须贯彻执行。
The hen cackles when she has laid an egg. 母鸡下蛋就咯咯地叫。
When you have washed the car , wipe it down well . 你把汽车冲洗过后，要好好用布揩拭一下。
Don't forget to switch off when you have finished. 做完之后别忘了关掉。
Put the book back when you have done with it. 你读完这本书时把它放回原处。
2. 条件状语从句：if, once, as long as
If you have finished reading the book before I leave, please lend it to me. 如果你在我离开之前就读完了这本书，请借给我。
If he has written the letter, I'll post it. 如果他写完了信，我就把它寄走。
Once we have promised, we should keep it. 一旦许下诺言，我们务必遵守。
We cannot move him from any decision once he has made it. 他一旦做出决定，我们就无法使他改变。
You can watch TV as long as you have finished your homework. 只要完成了作业，你就可以看电视。
As long as you have passed the previous three relevant practice game, we can end the confrontation between rivals. 只要你通过了前面三关的练习赛，就可以对决最终的对手。
3. 偶尔还用在让步状语从句中，如：
Even though you have set the bird free, it can hardly survive. 就算将来你放了这只鸟，它也很难活。
【注意】
1. 在第2句中，主句部分是个祈使句，实际上相当于一个一般将来时的句子，因为祈使句的动作都是将来才发生的。例如：
Don't forget to switch off when you have finished. 做完之后别忘了关掉。
Put the book back when you have done with it. 你读完这本书时把它放回原处。
Stay in the house until the danger has passed. 躲在家里，直到危险过后再出来。
I'm very busy just now; wait until things have eased up a little. 我现在非常忙，等事情缓一缓再说。
Don't drink the water until it has been cooled. 直到水冷了之后再喝。
2. 此外，在以上各个从句中，我们可以用一般现在时代替现在完成时，意思差别不大。
陈老师：
I’ll go home as soon as I have finished my homework.
I will go home as soon as I finish my homework(better)
Please lend me that book if you have finished reading it.
Please lend me that book if you finish reading it .(better)
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