各位老师好，大家训练册上有两道题目：
1.Frank will return the book if he has finished reading it.
2.The project will not start until all thepreparations have been made.
请问这两个句子为何填完成时，特别不懂第一个，虽然书上说在条件句里目前完成时可以替换以后完成时，但不了解为何用目前完成时，难道不能用一般目前时主将从现吗？
1.Frank willreturn the book if he has finished reading it.
2.The project willnot start until all the preparations have been made.
在这两个句子中，从句都可以用一般目前时：
1.Frank willreturn the book if he finishes reading it.
2.The project willnot start until all the preparations are made.
在“时间、条件状语从句” 中，虽然是以后的动作，大家也不可以用以后时，而是用一般目前时。这就是老师常常讲的所谓的 “主将从现”。同理，目前完成时用在“时间状语从句”或者“条件状语从句”中，表示以后业已完成的动作或状况，代替以后完成时。比如：
1. 时间状语从句：until, as soon as, when
We can't move inuntil the electricity has been laid on. 大家得等电源接通后才能搬进来。
I'll go to mygranny's as soon as I have finished my homework. 我一做完作业就去奶奶家。
Don't forget toswitch off when you have finished. 做完之后别忘了关掉。
2. 条件状语从句：if, once, as long as
If you havefinished reading the book before I leave, please lend it to me. 假如你在我离开之前就读完了这本书，请借给我。
once we havepromised, we should keep it. 一旦许下诺言，大家务必遵守。
You can watch TVas long as you have finished your homework. 只须完成了作业，你就能看电视。
3. 偶尔还用在让步状语从句中，如：
Even though youhave set the bird free, it can hardly survive. 即使以后你放了这只鸟，它也非常难活。
【说明】
（1）在以上每个从句中，大家可以用一般目前年代替目前完成时，意思差别不大。
（2）在时间状语从句中，用目前完成年代替以后完成时。这跟所谓的“主将从现”是一个道理。
（3）在时间状语从句和条件状语从句中，用目前完成时表示以后业已完成的动作。