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：
He had studied Chinese before he came to Taiwan.
He had worked before he decided to go to college.
I had been to Ireland before I wrote this book about Ireland's people.
I had studied calculus before I got into college.
It had already snowed before Monday.
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 。
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.