英语中，be动词的过去完成时态构造简单，只需在be动词的过去式had后加上过去分词。它主要用来表达过去某个时间点或某个动作之前已完成的动作，也可用于虚拟语气的条件句中表示虚拟的过去时间。掌握其构造和用法，能更准确地表达过去完成的动作和时间点。
过去完成时态的构造
过去完成时态的构造非常简单，只需要在be动词的过去式had后面加上过去分词即可。例如：
- I had been to London before I moved to New York.
- They had been talking for hours before I joined the conversation.
过去完成时态的用法
过去完成时态主要用来表达过去某个时间点或者某个动作之前已经完成的动作。具体来说，它可以用来表示以下几种情况：
1. 表示过去某个时间点之前已经完成的动作
- By the time I arrived at the party, everyone had already left.
- She had finished her homework before her parents came back home.
2. 表示过去某个动作之前已经完成的动作
- I had read that book before I saw the movie.
- They had already eaten lunch before I arrived.
3. 用于虚拟语气的条件句
在虚拟语气的条件句中，过去完成时态可以用来表示虚拟的过去时间。
- If I had known you were coming, I would have baked a cake.
- If she had studied harder, she would have passed the exam.
