真可-
before泛指过去
ago前面要有一个具体的时间，比如说one year ago
ago的参照点是"现在",常用于一般过去时；before的参照点是"过去的某个时间点"，常用于过去完成时。
before与ago用法比较
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，以副词的形式置于句末，可以用在现在完成时态中。例如：
①I have never been to Beijing before.
②Have 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.
②I 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。ago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“will / would +V.”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before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until。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。
9. before在句中的含义比较灵活，虽然before引导的也是一个时间状语，但译成汉语时却不必死译为“在……以前”。可以采取下面几种译法：（A）译为“……才”；（B）译为“……就”；（C）译为“尚未”、“还没有”、“来不及”、“不要”、“别”；（D）译为“在……的时候还没有”；（E）译为“如果”、“否则”、“免得”；（F）译为“到”、“等到”、“没等……就”；（G）译为“然后”等。例如：
A. It will be five years before we meet again. 要等五年我们才能再见面。
We waited a long time before the train arrived. 我们等了很长时间火车才到达。
B. I hadn"t waited long before he came. 我没等多久他就来了。
He had not gone a mile before he felt tired. 还没走一英里路，他就觉得累了。
C. His father died before he was born. 他尚未出世他父亲就去世了。
Catch him before he escapes. 抓住他，别让他跑了。
Take this computer before you change your mind. 买了这台电脑，不要三心二意了。
D. We arrived there before it began to rain. 我们到达那里的时候还没下雨。
E. Go before I call the police. 走吧，否则我叫警察了。
I had better go now before it is too late. 我最好现在就走，免得太迟。
Pigs will fly before he"ll be a scientist. 如果他当科学家，猪都会飞了。
F. The crops will die before the rain fall. 等到天下雨，庄稼早枯死了。
Before I could sit down, he offered me a cup of tea. 没等我坐下，他就给我端上一杯茶。
G. I shaved before I went to the party. 我刮了脸，然后去参加宴会。
马老四-
ago指 到现在为止，后面不能再添加特定时间；
before 后面可以再跟 时间名词
wpBeta-
before+时间状语
e.g:Before I walk into this room,He has eaten all the food.
ago只是时间语缀
e.g:A moment ago,the rain falls down.
cloud123-
before 可以引导一个时间状语从句，而ago却不可以