英语语法中，过去完成时是用来描述在过去某个时间点之前已经完成的动作或事件。构建过去完成时需要使用助动词"had"和过去分词。在一些情况下，我们可以使用"before"引导一个子句来构建过去完成时。以下是一些使用"before"构建过去完成时的例子：
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我们已经吃过晚饭了，因为我们在晚饭前已经吃了午饭。
- We had already eaten dinner because we had eaten lunch before dinner.
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她已经完成了作业，因为她在看电影之前已经做完了。
- She had finished her homework because she had done it before watching the movie.
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他们已经离开了，因为他们在我们到达之前已经离开了。
- They had left because they had left before we arrived.
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警察已经抓住了罪犯，因为他们在他逃跑之前就已经开始了追捕。
- The police had caught the criminal because they had started chasing him before he ran away.
通过使用"before"引导的子句，我们可以更加清晰地表达过去完成时的概念。不过，需要注意的是，在使用"before"构建过去完成时的句子时，要确保上下文的连贯性和语法正确性。
