在英语语法中,since是一个引导词,用于表示某个动作或事件的开始时间。在过去完成时中,since通常和have/has结合使用,表示在过去某一时刻开始,一直持续到现在的时间。
以下是since引导的过去完成时的例句:
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我们已经等了两个小时,因为公交车从下午一点钟就离开了。 We had been waiting for two hours since the bus left at 1 pm.
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小明已经去了美国一年了,因为他在那里学习。 Xiao Ming had been in the United States for a year since he was studying there.
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警察已经在犯罪现场调查了两个小时,因为他们到达时罪犯已经逃跑了。 The police had been investigating the crime scene for two hours since the criminal had fled when they arrived.
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我已经了解这个问题两个星期了,因为我在这方面进行了研究。 I had been aware of the issue for two weeks since I had researched it.
通过以上例句,我们可以看出since引导的过去完成时主要用于描述一个时间段的开始时间,并强调这段时间一直持续到目前为止。它通常用于叙述过去的事件,以及事件与当前的关系。希望这些例句可以帮助你更好地理解和运用since引导的过去完成时。