过去完成时的常见用法：

一。过去完成时表示一个动作先于另一个动作的发生，更多见于宾 语从句中。若主句是一般过去时，且谓语动词为decide; discover;find ;realize;think; forget; ect.

Eg: 1) They realized that they had lost their way. 2) I found that she had missed the train.

二。在间接引语中，通常用过去完成时表示说话者在说话时动作 已完成。

Eg:1)She said that he had learned advanced mathematics all by himself. 2)They told me that they had known each other for three years.