【一】earlier 用法解析

(1)earlier是earlier比较级，和than连用有比较的味道，意思是“比....早/先于”

I came here 10 minutes earlier than you。

我早你10分钟到的。

(2) 时间段和earlier 一起使用表示是以过去或将来某一个时间点为准的一段时间以前(英文解释： Earlier is used to refer to a point or period in time before the present or before the one you are talking about.)

a few hours earlier than the time when the detectives were waiting at the airport.

在侦探们等在飞机场之前的几个小时以前.

When I got to school yesterday. I didn't see Jim. But one hour earlier, he told me on the phone he had been there.
当我到学校的时候,我没有看到Jim.可是(在我到学校)一个小时前,他在电话里说他在那里
(3)表示动作发生在早些时候;先前;之前(英文解释： Earlier is used to refer to a point or period in time before the present or before the one you are talking about.)

 As mentioned earlier, the University supplements this information with an interview...

如前所述，大学将通过面试对这一信息进行补充。

Earlier, it had been hoped to use the indoor track.  

之前曾经希望能使用室内跑道。

【二】ago 用法解析

它描述动作发生的间是从现在算起，即以现在时间为起点算起之前，句子只能能用一般过去时 (英文解释：You use ago when you are referring to past time. For example, if something happened one year ago, it is one year since it happened. If it happened a long time ago, it is a long time since it happened.）

He was killed a few days ago in a skiing accident.

几天前他在一场滑雪事故中丧生。

A short while ago, my sister helped me to carry one of my old bookcases up the stairs.

片刻之前，我妹妹帮我把我的一个旧书柜抬上了楼。

It was stolen twenty years ago when Ted was a boy of fifteen.

那辆自行车是在20年前当泰德还是个15岁的小伙子时被偷去的。

Now I am at home. But I was in the office three hours ago.

现在我在家里，但是3小时之前我在办公室
:Oh?Why are you here so soon?I saw you in the market ten minutes ago.

你怎么这么快就来了?十分钟前我还看到你在市场了呢.

备注：在直接引语变间接引语，ago要改成before

He told me," I was here three days ago.

他告诉我，“我3天前在这里”

"改写成:He told me that he had been there three days before.

他告诉我他3天前在这里。
【三】before用法解析

它用于过去完成时,它所指的时间不是从现在算起，而是从过去某一时刻算起，有时可以与previously换用

 A short time before, great trees had covered the countryside for miles around.

在那时不久之前，高大的树木覆盖着方圆数英里的地方。

Last summer, I finally left the firm that I had joined eighteen years before.

去年夏天，我终于离开了从那时算起18年前我所参加的那个商行。

备注：有时before=earlier 

A few hours earlier, someone had told the police that thieves would try to steal the diamonds. 

几小时之前，有人已告诉警察说，小偷打算盗窃那些钻石。

before还有下列搭配用法

(1)如果强调从句动作发生之前，主句动作已发生，可译成“未……就”或“还没有……就”。

 Before I could get in a word he had measured me．

我还没有来得及插话，他就给我量号了尺寸。

 Lu Ban built nine pavilions, but before he could finish the tenth, he heard a cock crowing．

鲁班建了九个亭子，但他还没完成第十个就听到公鸡叫了起来。

(2)有些句子中的before可译为“先……然后”、“先……再”。

Don’t count the chickens before they are hatched． 

不要乐观得太早。

 Be pupil before you become a teacher．

先做学生，再做先生。

(3)有些句中的before还可译为“在……内”、“趁”、“没”。

They arrived before I expected．

我没想到他们到得那么早。

Study hard before it is too late．

趁早努力学习。

I had better go now before it is too late．

最好我现在就走，免得太迟．

I will be through this book before three days have passed．

我要在三天内看完这几本书．

 (4)before sb．knew it 一类习惯说法，常译为“不知不觉……就”、“还没弄清

就”、“不知怎么地……就”。

Time passed quickly and three months went by before Aqiao knew it．

时间飞快的流失，阿巧不知不觉地在那里度过了三个月。

He fell from the tree before he knew it．

他不知怎地一下就从树上掉了下来。 

    【答】Only a month earlier, Mr. Stenholm had declared for the tax cut.

就在一个月前，斯滕霍尔姆先生刚刚表示过赞成减税。

   这句中的 earlier 可以换成 before ，两者没有什么区别。