什么时候该用现在完成时,又什么时候该用过去式?

什么时候该用现在完成时,又什么时候该用过去式?
什么时候该用现在完成时,又什么时候该用过去式?

什么时候该用现在完成时,又什么时候该用过去式?
1.现在完成时用来表示之前已发生或完成的动作或状 态,其结果的确和现在有联系.动作或状态发生在过去 但它的影响现在还存在；也可表示持续到现在的动作或 状态.其构成：have (has) +过去分词.
2过去进行时的基本用法
a.过去进行时表示过去某一时间正在进行的动作,常和表过去的时间状语连用,如：
I was doing my homework at this time yesterday.昨天的这个时候我正在做作业.
They were expecting you yesterday.他们昨天一直在等待.
b.过去进行时可与soon,the next moment,in minutes,minutes later等时间状语连用,表示一个新的
动作刚刚开始.如：
Soon the whole town was talking about it.不久镇上的人就都谈论起这种事了.
c.过去进行时可用来申述原因或用作借口,这种用法常用在口语中.如：
-- Have you finished your homework,Mary?玛丽,你作业做完了吗?
-- No,I was helping my mother is the kitchen all day yesterday.还没呢,我昨天一天都帮妈妈在厨
房干活.
d.过去进行时可用来为一个后一系列动作的发生提供背景.如：
I hurt my leg when I was riding a bike.我在骑车时把腿摔坏了.
e.过去进行时可表示过去未曾实现的愿望或打算,这时be动词was/were要重读.如：
I was writing him a letter this morning and forgot all about it .我本该今天早上给他写信的
,后来全给忘了.
I was seeing her tomorrow.我本来打算明天会见她.
He was watching the play yesterday,but he was too busy.他昨天本来要看那场戏的,可是太忙
了.
过去进行时表示过去某一段时间或某一时刻正在进行的动作.常与之连用的时间状语有,at that
time/moment,(at) this time yesterday (last night/Sunday/week…),at+点钟+yesterday (last
night / Sunday…),when sb.did sth等时间状语从句.
What were you doing at seven p.m.yesterday?昨天晚上七点你在干什么?
I first met Mary three years ago.She was working at a radio shop at the time.我第一次遇到玛丽
是在三年前,当时她在一家无线电商店工作.
I was cooking when she knocked at the door.她敲门时我正在做饭.
（3）一般过去时往往表示某一动作已经完成,而过去进行时却表示动作在持续或未完成.如：
I saw you while you were speaking to the teacher.你在和老师谈话时我看见了你.
注意：有的过去时间状语既可用于一般过去时,也可用于过去进行时,但含义不同.如：
She wrote a letter to her friend last night.她昨晚给她的朋友写了封信.（信写完了）
She was writing a letter to her friend last night.她昨晚一直在给她的朋友写信.（信不一定写完）
一、过去进行时的时间状语
1.when 和while引导的状语从句中,强调某个动作正在进行的过程中时要用进行时态,while表示一段时
间,因此它所引导的状语从句中,谓语动词常用进行时态,如：
When/While we were having supper,the light went out.我们正在吃饭时灯熄灭了.
2.when用作并列连词时,主句常用进行时态,从句则用一般过去时,表示主句动作发生的过程中,另一个意
想不到的动作发生了.如：
I was walking in the street when someone called me.我正在街上走时突然有人喊我.
二、下面几种情况不用一般过去时而要用过去进行时：
1.表示过去某一阶段暂时性的习惯动作时.如：
Tom was getting up at six o’clock every day that week.汤姆那一周里每天都是六点钟起床.
2.与always连用表示赞美,厌烦等感情色彩时.如：
John was always coming to school late.约翰上学总是迟到.
Lei Feng was always doing good deeds for the people.雷锋总是为人民做好事.
3.用来描写故事发生的情景时.如：
It was a dark night.The wind was blowing hard and the rain was falling heavily.A PLA man
suddenly appeared on the river bank.He wanted to cross the river.那是一个漆黑的夜晚,风刮得很
厉害,雨下得很大,一个解放军战士突然出现在河岸上,他想过河去.
4.when作并列连词,表示“（这时）突然”之意时,第一个并列分句用过去进行时,when引导的并列分句
用一般过去时.如：
I was taking a walk when I met him.我正在散步,突然遇见了他.
We were playing outside when it began to rain.我们正在外边玩,这时下起雨来了.
5go,come,leave,start,arrive等动词可用过去进行时表示过去将来的含义.如：
I was leaving for Wuhan that day.那天我正要去武汉.
She was coming later.她随后就来.
三、过去进行时表示婉转语气（只限于want,hope,wonder 等动词）,用以提出请求.如：
I was wondering if you could help me.
I was hoping you could send me home.

现在完成时：过去的事情现在还有后果，一般有这几个词：already yet before never since （ever since）for （how long）等
过去式就是过去的事：the day before yestoday
last night等过去的时态
一般要自己去判断