ago, before作副词时有何不同的用法?

作副词时，都有“以前”的意思。其用法区别如下，

1. ago和一般过去时连用，表示现在以前的时间，一般不用于完成时态，但可用在情态动词加完成时不定式结构里，表示现在对过去某事的推测和判断。例如：

Faber must have seen her a month ago.费伯一个月前一定见过她。

2. before表示在早先或过去某时间的“以前”，也可泛指“从前”，用于在完成时、过去完成时或过去时。例如：

I have read it before.

I didn't know that before.

比较：

Herbert said that his mother had died five years before. (距他说话时五年)

His mother died five years ago. (距现在五年）

当before用作连词时，有如下几种用法：

1.“在……之前”。例如：

We do want to buy something now before prices go up.在物价

涨之前，我们确实想买点东西。

2. 引深为“与其……宁可……”。例如：

He will starve before he will steal.他宁可饿死，也不愿偷。

3.“…(之后)…”，主句一般是肯定句。例如：

It will be five years before we meet again.五年后我们才会重新见面。

It was evening before we reached the little town.到晚上我们才到达那个小镇。

4.“(不多久)就…”，主句一般是肯定句。例如：

It was not long before he told us about this affair.不多久他就给我们谈了此事。

I hadn't waited long before he came.(=He came a short time after

I had begun to wait for him.) 我没等多久他就来了。