People often remember what they ( ) (do)wnen they heard the

People often remember what they ( ) (do)wnen they heard the news of important events in history.
用适当形式填空.请说明为什么.

答案：have done
之前做过的事情,后果延续至今（又被现在提起）；所以时态要用现在完成时.

解析：不用 had done 原因：
often remeber 对应 have done ；一般现在时对应的也应该是现在完成时, 一般过去式则对应的是过去完成时 remembered 对应 had done.

通常使用 had done 的情况为以下几种：

* When it's important to say which event happened first in the past 在过去曾经发生的重大事情,通常为战争或者灾难.
有些时候我们需要强调已经发生过的事情的先后顺序,需要用使用主动发生的”had done“或者被动发生”had been done“比如：
"I was hungry, as I hadn't eaten all day." (Being hungry happened after not eating all day.)
Compare these two sentences:
"When he got to the airport, the plane took off." (The plane took off after he arrived at the airport.)
"When he got to the airport, the plane had taken off." (The plane took off before he arrived at the airport.)

通常have done的表达形式为下列3种.

* In reported speech 演讲的时候诉说历史性事件或提到曾经发生过的事情.
"I have never been to Scotland."
She said that she had never been to Scotland.

* To show regret about the past 对曾经的事情表示惋惜和悔恨的时候
I wish I hadn't gone to the party. (But I went.)

* In conditional (if) sentences 在以”if“引导的条件状语从句中.

If only she hadn't bought that car. (But she did.)

If I had worked harder, I would have passed the exam. (But I didn't work harder, so I failed.)

If you had told me, I could have helped. (But you didn't tell me, so I couldn't help.)

If they had left earlier, they might have caught the plane. (But they didn't leave earlier, so they didn't get the plane.)

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