怎样区分过去完成时和现在完成时

怎样区分过去完成时和现在完成时
过去完成时是had+V
现在完成时是have has +V
但什么时候用过去完成时什么时候用现在完成时

概念：表示过去的过去
----|-------|-----|---->其构成是had +过去分词构成.
那时以前 那时 现在
2） 用法
a.在told,said,knew,heard,thought等动词后的宾语从句.
She said (that) she had never been to Paris.
b.状语从句
在过去不同时间发生的两个动作中,发生在先,用过去完成时；发生在后,用一般过去时.
When the police arrived,the thieves had run away.
c.表示意向的动词,如hope,wish,expect,think,intend,mean,suppose等,用过去完成时表示原本…,未能…
We had hoped that you would come,but you didn't.
3) 过去完成时的时间状语before,by,until ,when,after,once,as soon as.
He said that he had learned some English before.
By the time he was twelve,Edison had began to make a living by himself.
Tom was disappointed that most of the guests had left when he arrived at the party.
典型例题
The students ___ busily when Miss Brown went to get a book she ___ in the office.
A.had written,left B,were writing,has left C.had written,had left D.were writing,had left
答案D.把书忘在办公室发生在去取书这一过去的动作之前,因此忘了书这一动作发生在过去的过去,用过去完成时.句中when表示的是时间的一点,表示在同学们正忙于……这一背景下,when所引导的动作发生.因此
前一句应用过去进行时.
注意：had no … when 还没等…… 就……
had no sooner… than 刚…… 就……
He had no sooner bought the car than he sold it.
11.16 用一般过去时代替完成时
1) 两个动作如按顺序发生,又不强调先后,或用then,and,but 等连词时,多用一般过去时.
When she saw the mouse,she screamed.
My aunt gave me a hat and I lost it.
2 ) 两个动作相继发生,可用一般过去时；如第一个动作需要若干时间完成,用过去完成时.
When I heard the news,I was very excited.
3) 叙述历史事实,可不用过去完成时,而只用一般过去时.
Our teacher told us that Columbus discovered America in 1492.
再问： 能不能稍微精简一些，主要的分别是什么？？
再答： 就看动作是发生是之前还是之后啊