知识点1:现在完成时转换成一般过去时
现在完成时可以转化成一般过去时，还可以转化成“It is/has been +时间段+since从句(用一般过去时)”等句型。
e.g. Jim has been back for a month. 吉姆已经回来一个月了。 = Jim came back a month ago. 吉姆一个月前就回来了[1]back,came,相关知识点

在状语从句中要用一般时来表示将来，如：I should tell him when he came back. ［误］ I'll come to see you as soon as I'll be back. ［正］ I'll come to see you as soon as I am back. ［析］ 在状语从句中要用一般时来表示将来，如：I should tell

= It is/has been a month since Jim came back. 自吉姆回来已经有一个月了。 = A month has passed since Jim came back. I have kept the book for two days. = I borrowed the book two days ago.[2]since,been,相关知识点

It's/has been +一段时间+since从句 自从某时起做某件事情已经一段时间了。　　该句型中since引导的时间状语从句常用一般过去时。例如：　　It's twenty years since he came here. 他来这里已经20年了。　　It has been six years since he married Mary. 他和玛丽结婚已经六年了。考试用

= It is / has been two days since I borrowed the book. = Two days has passed since I borrowed the book.[3]since,been,相关知识点

知识点1: IT +IS / HAS BEEN（WAS） + 时间段 +SINCE 从句It +is / has been （ was ） + 时间段 +since 从句在这一结构中，主句常用一般现在时、现在完成时或一般过去时。如果 since 引导的状语从句中的谓语动词为非延续性动词，则表示“自从状语从句中的动作发生以后，时间过不了多久”。例如：[1]

知识点2:现在完成时与一般过去时的区别

现在完成时与一般过去时的区别
现在完成时表示的是与现在有关联的过去事件；而一般过去时则与现在毫无关系，只是单纯的过去的情况。比较：
I have lost my pen, so I have to buy one. 我丢了钢笔，所以得去买一枝。[4]have to,have,相关知识点

have go还是have to gohave go还是have to go请看下面的题，是选have go还是have to go?Whom would you rather _________ with you, Jim or Jack?A. have go B. have to go C. have gone D. has to go有的同学从一看选项便首先排除了A，认为

I lost my pen, but found it later. 我丢了钢笔，但后来找到了。
当句子中有表示明确过去的状语时，只能用一般过去时，不能用现在完成时。如：
他一周以前动身去了纽约。
误：He has left for New York a week ago.
正：He left for New York a week ago. [5]New York单词记忆法

◆记忆方法一：联想方式：New+Y（衣杈）+or+k（机关枪）去纽约市带上新的衣杈还是机关枪◆记忆方法二：联想方式：New+Y（衣杈）+or+k（机关枪）记忆方法：去纽约市带上新的衣杈还是机关枪

另外，当句首为疑问词when时，其后可用一般过去时，不能用现在完成时。如：
你什么时候和他首次见面的?
误：When have you first met him?
正：When did you first meet him?