一、时间先后关系
1. By the time we arrived at the station, the train had already left. （我们到达车站时，火车已经离开了。）
2. The movie started when we were sitting in the cinema. （我们正坐在电影院时电影已经开始了。）
二、结果或影响
1. The cake was already baked when you called. （你打电话时蛋糕已经烤好了。）
2. He had finished his project before the deadline, so he had time to relax. （他提前完成了项目，所以有时间放松。）
三、被动语态
1. The letter had been delivered by the time we read it. （我们读信时信已经被投递了。）
2. The problem had been solved by the expert team. （问题已经被专家团队解决了。）
四、条件句
1. If I had known you were coming, I would have baked a cake. （如果我知道你要来，我就烤蛋糕了。）
2. She would have finished her homework if she hadn't been distracted. （如果她没有被分心，她就会完成作业了。）
五、含否定句
1. He did not （had not） finished his homework when the teacher called. （老师打电话时，他还没有完成作业。）
2. We had not arrived at the meeting when the guest arrived. （客人到达时，我们还没有到达会议现场。）
补充说明：
过去完成时与过去简单时的区别：过去完成时强调“过去的过去”，即一个动作在另一个过去动作之前完成；过去简单时仅表示过去某个时间点的动作。
常见错误：将过去完成时误用为一般过去时，例如：He said he had worked in the factory since 1949.（正确）而非He said he worked in the factory since 1949.（错误）。
建议结合具体语境选择合适时态，并通过练习加深理解。