过去完成时态用来描述在过去某个时间点之前已经完成的动作或事件。
以下是一些例句：
1. I had already finished my homework before my friends arrived.
这句话指的是在我的朋友到来之前，我已经完成了作业。
2. She had traveled to several countries before she turned 30.
这句话意思是在她30岁之前，她已经去过几个国家旅行了。
3. They had already eaten dinner when I arrived home.
当我到家时，他们已经吃完晚饭了。
4. By the time we got to the cinema, the movie had already started.
我们到达电影院时，电影已经开始了。
5. He had read all the books in the library before the new ones arrived.
新的书到来之前，他已经读完了图书馆里的所有书。
总之，过去完成时态用来表达在过去某一时间点之前已经完成的动作，通过使用had + 过去分词的结构。