现在完成时表示到现在为止，已经完成的动作或存在的状态。在时间状语从句中用现在完成时表示将来要完成的动作
过去完成时表示到过去某一时间或动作为止，已经完成的动作或存在的状态。在表示过去没有实现的愿望、想法等意思时，要用过去完成时。
将来完成时则表示到将来某一时间或动作为止，将要完成的动作或存在的状态。
例如：
We have learned 2,500 English words by now.
I’ll go on a trip to Si’an when I have finished writing this book.
We had learned 2,000 English words by the end of last term.
I had hoped to visited you last weekend ,but I was too busy then.
We will have learned 35,000 English words by the end of next month.
练习：