I had already finished my homework when my mother came back.

He had come back from the supermarket before 5 o'clock.

I had finished my homework before supper.

The play had already started when we got to the theatre.

By the end of June, they had treated over 10000 patients.

He said he had worked in that factory since 1949.

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

By the end of last term, we had learned 5000 new words.

She had already eaten dinner.

We have explored Shanghai.

这些例句展示了过去完成时在不同句子结构中的使用，以及它如何用来描述在过去某个时间点之前已经完成的动作或状态。