第四章 完成式（perfect tense）
4-3 过去完成进行式（past perfect progressive tense）
过去完成式的形式：had + past participle
过去完成式是不能单独用的。我们用的时候，必须有另一个事件。也就是说，假如我们有两件事 a 和 b，两件事都发生在过去，但 a 发生在 b 以前，a 应该用过去完成式，b 则用过去式。以下是几个例子：
（1）他到台湾以前，曾学过中文。
he had studied chinese before he came to taiwan.
（2）他念大学以前，曾前工作过。
he had worked before he decided to go to college.
（3）我写这篇有关爱尔兰的小说以前，曾去过爱尔兰。
i had been to ireland before i wrote this book about ireland's people.
（4）我在上大学以前，已经学过微积分。
i had studied calculus before i got into college.
（5）周一以前，已经下过雪了。
it had already snowed before monday.
如果我们要强调较早发生事件的连续性，我们可以用过去完成进行式。
过去完成进行式和现在进行式惟一不同的地方是 verb to have 的地方一定要用had 。
以下是一些过去完成进行式的例子：
1.i had been watching tv before you called me.
2.i had been working hard in a company for many years before i went to college.
3.he had been studying before he went to class.
4.he had been driving all day before he went to sleep.