过去完成时是英语中表示过去某个时间点或事件之前已经完成的动作的时态。在使用过去完成时时，除了需要正确的时态形式，还需要使用一些标志性的词语来表达动作完成的时间点。以下是过去完成时21个标志性词语。
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By the time（到...时）
- By the time he arrived, we had already eaten dinner.（他到达时，我们已经吃过晚饭了。）
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Before（在...之前）
- Before I left the office, I had finished all my work.（在我离开办公室之前，我已经完成了所有的工作。）
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After（在...之后）
- After I had finished my homework, I watched TV.（在我完成作业之后，我看了电视。）
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When（当...时）
- When she arrived, we had already left.（当她到达时，我们已经离开了。）
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As soon as（一...就...）
- As soon as he had finished his coffee, he left.（他一喝完咖啡就离开了。）
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Until/till（直到...）
- I didn't go to bed until I had finished reading the book.（我直到读完这本书才去睡觉。）
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Since（自从...以来）
- I had been waiting for her since 3 o'clock.（我从3点开始就一直在等她。）
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Just（只是...而已）
- I had just finished my meal when the phone rang.（我只是刚吃完饭电话就响了。）
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Already（已经）
- They had already left when I arrived.（当我到达时，他们已经离开了。）
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Yet（还没有）
- I had not yet finished my homework when my friends came over.（当我的朋友来时，我还没有完成我的作业。）
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Finally（最终）
- I had finally won the game after many attempts.（经过多次尝试后，我最终赢得了比赛。）
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Recently（最近）
- I had recently moved to a new city before I started my new job.（在我开始新工作之前，我最近搬到了一个新城市。）
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Lately（近期）
- Lately, I had been feeling tired all the time.（近期，我一直感到疲倦。）
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So far（到目前为止）
- So far, I had visited five countries.（到目前为止，我已经去过五个国家。）
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By then（那时）
- By then, I had already made up my mind.（那时，我已经下定决心了。）
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Up to that point（到那个时候为止）
- Up to that point, I had never traveled alone.（到那个时候为止，我从未独自旅行过。）
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Already（已经）
- They had already started the movie when we arrived.（当我们到达时，他们已经开始看电影了。）
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Yet（还没有）
- I had not yet finished my work when my boss called me.（当我的老板给我打电话时，我还没有完成我的工作。）
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Before（在...之前）
- Before I could say anything, she had already left.（在我说什么之前，她已经离开了。）
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Until/till（直到...）
- I waited until she had finished talking before I responded.（在她说完之前，我一直等待着。）
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Had just（刚刚...）
- I had just left the house when it started raining.（我刚出门就开始下雨了。）
使用这些标志性词语可以更好的表达过去完成时态，帮助我们清晰地传达动作完成的时间点和顺序。
