末速度before／ago,since用法

2021年1月19日发(作者：野餐)
before
和
ago
都能表示在一段时间以前，但是二者的内涵和具体用< br>法有明显的区别：




1. before
表示 从过去或将来某一时间算起多少时间以前，由于与这一时间状
语连用的行为先发生在过去或将来某一时刻 之前，
因此往往出现在过去完成时态
或将来完成时态中。例如：




①

I called him up last night
，

but his mother told me he had left for Wuhan
two days before.



②

She said that she had married him five years before.



2.
如果不具体表 明多少时间以前，
before
意为

从前；以前

，表示从现 在开始
追溯到过去一段时间，
或者主句的谓语动词表示的是经常、
反复发生的行为，< br>以
副词的形式置于句末，可以用在现在完成时态中。例如：




①
I have never been to Beijing before.



②
Have you ever seen a tiger before?



③

I often go over the lessons that I have learned a few days before.



注意：

before
前的时间状语如果是特定的，含义不一样。试比较：




He said he had arrived here a day before. (
一天前
)



He said he had arrived here the day before. (
前一天
)



3. ago
指的是从现在起到过去时间的某一点的一段时间 。
这个时间状语常与一
般过去时态或过去进行时态连用。例如：




①

I graduated from the college twenty years ago.



②
I was asked what I was doing a month ago.



4.
表示在某一时间点或事件以前时，只能用
before
，不用
ago
。因为这种用
法是将
before
当作介词或连词使用。
ag o
不具有这种功能。例如：




①

He will come back before eleven o'clock.



②

Where did your brother study before he joined the army?



③

It was hoped that this would be achieved before the year 2006.



5.
按语法规则，
ago
只能与一般过去时连用，但在
since three years ago
这类
短语中能与现在完成时态连用。因为
three years ago
构成一个名词短语，共同
作
since
的宾格，意为
自从三年前以来

。例如：




①

He has been here since three years ago.



②

I have taught English in this school since twenty years ago.



6.
含
before
的从句一般不用将来 时态，
如果采用

时，
一般被
认为是情态或虚拟语气。例如：




I would die before I would confess to it.



7.
如果
befo re
从句是一般过去时态，所暗示的是过去发生的事，主句则宜用
过去完成时，
语义偏 重于过去某阶段的行为或状况时，
还可与一般过去时态连用。
例如：




①

I had never seen a Red Army man before I arrived in Xi'an.



②

They asked me things that happened before the boy was born.



8.
不少的语法书上说，
before
引导的从句习惯上不与否定的主句连用，要用
un til
。实际上二者都可以，有时在意义上有些区别。试比较：




He didn't arrive until I returned.
直到我回来时他才到达。




He did not arrive before I returned.
在我回来之前他尚未到达。




I didn't manage to do it until you had explained how.
直到你解释了如何干，
我才设法完成了那项工作。




I didn't manage to do it before you had explained how.
你解释了如何干之
后，我才设法完成了那件工作。