一般说来，现在完成时表示过去发生的动作对现在产生的影响和结果，着重点在现在。过去完成时表示过去某一时间或动作之前发生的另一动作或状态，即“过去的过去”，必须有一个具体的过去时间或动作来支撑，强调对比。一般过去时只表示过去发生了某一动作，与现在没有联系，并且有明显的时间间隔。
例如：
1.He had learned more than 5,000 English words when he entered the university at the age of 15?
2.Years ago we didn't know this, but recent science has shown that people who don't sleep well soon get ill.
3.The policeman's attention was suddenly caught by a small box which had been placed under the Minister's car.
4.When the old man started to walk back to his house, the sun had already hidden itself behind the mountain.
5.He was hoping to go abroad but his parents have decided that they won’t support him unless he can borrow money from the bank.
6.—
If the traffic hadn't been so heavy, I could have been back by 6 o'clock.
—
What a pity! Tina was here to see you.
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