英语的过去完成时表示过去某个动作发生在另一个过去动作之前，有三种常见句型：1) had + 过去分词，如"I had finished my homework before my mom came home."；2) had been + 现在分词，如"I had been watching TV for two hours when my friend called me."；3) had + 过去完成时，如"They had already left before we arrived at the party."。在学习英语过去完成时的时候，需要注意这三种句型的区别。
第一种句型：had + 过去分词
这种句型的结构是had + 过去分词。例如：
- I had finished my homework before my mom came home.（我在妈妈回家之前已经完成了作业。）
- He had studied English for two years before he moved to the United States.（他在搬到美国之前已经学了两年英语。）
第二种句型：had been + 现在分词
这种句型的结构是had been + 现在分词。例如：
- I had been watching TV for two hours when my friend called me.（当我朋友给我打电话的时候，我已经看了两个小时的电视了。）
- She had been working at the company for five years before she got promoted.（她在被提升之前已经在这家公司工作了五年。）
第三种句型：had + 过去完成时
这种句型的结构是had + 过去完成时。例如：
- They had already left before we arrived at the party.（我们到派对之前，他们已经离开了。）
- The movie had already started by the time we got to the theater.（我们到电影院的时候，电影已经开始了。）
