现在完成时，用在时间状语从句或条件状语从句中，表示将来业已完成的动作或状态，代替将来完成时。例如：

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。

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。

Put the book back when you have done with it. 你读完这本书时把它放回原处。

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。

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。

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。【注意】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。