过去完成时的标志词有by、before、after、already等。
by是到……为止表示到某个时间为止已经完成的动作或状态。例如By the time I arrived at the airport，the plane had already taken off.我到达机场时，飞机已经起飞了。
before是在……之前表示在某个时间或动作之前已经完成的动作或状态。例如I had never seen such a beautiful sunset before I went to Hawaii.在去夏威夷之前，我从未见过如此美丽的日落。
already已经表示已经完成的动作或状态。例如I had already eaten breakfast by the time she called me.她给我打电话的时候，我已经吃过早餐了。
just刚刚表示刚刚完成的动作或状态。例如The train had just left when we arrived at the station.我们到站时，火车刚刚开走。
already（已经表示已经完成的动作或状态。例如He had already seen that movie before he went to the cinema.他去电影院之前已经看过那部电影了。
使用过去完成时的注意事项
1、过去完成时使用“had+过去分词”形式构成，表示在过去某一时间之前已经完成的动作或事件。例如：He had finished his homework before he went to bed.他睡觉前已经完成作业。
2、过去完成时常与过去简单时连用，表示在过去某一时间之前已经完成的动作或事件对过去某一时间的影响或结果。例如：When I arrived at the station，the train had already left.当我到达车站时，火车已经离开了。
3、过去完成时也可以用于虚拟语气中，表示虚拟的过去完成时，即假设过去某一时间之前已经完成了某个动作或事件。例如：If I had known the answer，I would have told you.如果我知道答案，我会告诉你的。
3、在使用过去完成时的时候，需要注意时态的一致性。如果主句是过去时态，从句则应使用过去完成时态。例如：He told me that he had finished his homework.他告诉我他已经完成了作业。