过去完成时（Past Perfect Tense）用于描述在过去某一时间或动作之前已经发生或完成的动作。其基本句型结构如下：
1. 肯定句：主语 + had + 过去分词
2. 否定句：主语 + had + not + 过去分词
3. 疑问句：Had + 主语 + 过去分词
4. 特殊疑问句：疑问词或词组 + had + 主语 + 过去分词
5. 被动语态：主语 + had （not） + been + 过去分词
例如：
By the time I arrived, they had already left. （我到达时，他们已经离开了。）
Before I left, I had finished my homework. （在我离开之前，我已经完成了我的作业。）
She had been studying English for five years before she moved to Canada. （在她搬到加拿大之前，她已经学习了五年英语。）
He had already eaten breakfast before going to work. （在他去上班之前，他已经吃过早餐了。）
这些句型可以帮助你构建过去完成时的句子，表达在过去某个时间点之前已经完成的动作或状态