过去完成进行时的延续性动词

在英语过去完成进行时中，例如 work、study、stay、sing、teach等延续性动词与表示一段时间的时间状语连用时，用过去完成进行时可以表示一个动作一直延续到过去某一时刻，通常强调动作的持续性和不间断性。例如：
1.She had been studying for two hours when her classmate came to visit her.
当她的同学来看她的时候，她学习有两个小时了。
2.I had been writing a letter for two hours by the time she came to see me.
当她来看我的时候，我已经写了两个小时的信了。

3.They had been having breakfast for ten minutes when Tom came in.
汤姆来时，他们已经吃了十分钟的早饭了。

4.She had been dancing for one hours when her mother came back home.
当她的妈妈回到家时，她跳舞有一个小时了。
5.I had been reading this book for three hours by the time she came to see me.
当她来看我的时候，我已经读了三个小时的书了。

6.They had been having class for twenty minutes when Tom came in.
汤姆来时，他们已经上了二十分钟的课了。

7.My father had been watching TV for two hours when his friend came to visit him.
当他的朋友来看他的时候，我的父亲已经看了两个小时电视了。

8.Tom had been playing piano for two hours by the time Lily came to see him.
当丽丽来看他的时候，汤姆已经练习钢琴两个小时了。

9.We had been talking about this project for one hour when manager came in.
经理来时，关于这个项目，我们已经讨论了一个小时了。