They seemed to be too nervous and were too anxious to leave, so I asked whether they wanted to go ahead of me or not. They didn’t understand what I meant/ had meant at first. After knowing it, they were very appreciative. But they stood behind me. 

这个句子是用过去完成时还是一般过去时。

didn't understand 是过去发生的，mean是understand之前的，所以用 had meant，这个理解对吗？    

【答】网友的理解完全正确。“过去完成时”的核心内容是“过去的过去”。记住了这个大前提，一切就会迎刃而解。

为了体现“过去完成时”是“过去的过去”这个原则，我编写了20个句子，每个句子都包含着“一般过去时和过去完成时”两种时态，请仔细体会：

结合下面的20个例句，请仔细体会“过去完成时”的用法，为什么使用“过去完成时”？如果改为一般过去时的话，句子意思和动作先后关系能不能看懂？

1. Tom was disappointed that most of the guests had left when he arrived at the party.

2. The children ran away because they had broken the window.

2. By the time he was twenty, he had written five novels.

3. How many English songs had you learnt by the end of last term?

4. As soon as the sun had set, we returned to our hotel.

5. How long had Brown lived in the south before he came here?

6. When he had eaten his lunch he asked for a glass of water.

7. I hadn’t understood the problem until the teacher explained it.

8. They had just got home when it began to rain.

9. I had been at the bus stop for 20 minutes when a bus finally came.

10. He said he had worked in that factory since 1949.

11. Mr. Smith died yesterday. He had been a good friend of mine

12. I didn’t know a thing about the verbs, for I had not studied my lesson.

13. I returned the book that I had borrowed.

14. I met Tom yesterday. We hadn't seen each other since he went to Beijing.

15. I thought I had sent the letter a week before.

16. When I woke up, it had already stopped raining.

17. We had hoped to be able to come and see you, but you were not at home.

18. It was the third time that he had been out of work that year.

19. We had learned over two thousand English words by the end of last term.

20. They had planted six hundred trees by last Wednesday.