They were here an hour ago.一小时前他们在这儿。
He said that his sister had been there three days before.他说他妹妹三天前来过这儿。
<辨析>这两个词都解作“以前”。ago 是副词，表示从现在算起的若干时间以前，谓语动词常用过去时。如：
He can ride a bike now,but he couldn't a few weeks ago.他现在会骑自行车，但几个星期前他不会。
before 也可用作副词，通常表示从过去某时刻算起的若干时间以前，谓语动词常用过去完成时。如：
When we finally got there,the team had left an hour before.我们最后到达那里时，小队已在一小时前离开了。
before 可用在现在完成时、一般过去时的句子里，不伴有具体的时间而单独使用，表示“笼统，不明确的以前”。如：
I have read the book before.我以前读过这本书。
I met that man before.我从前碰见过那个人。
before 还可以用作传记介词或连词，用作介词时，后接某点时间。如：
Please come to see me before six o'clock.请在六点以前来看我。
Where did you sister study before she joined the army?你妹妹参军前在哪里学习？
与单词：before 相关的例句或者名人名言
Everyone is equal before law.
法律面前人人平等。
Could he run fast enough to get home before the storm?
他能跑得够快赶在暴风雨来临之前到家么？
You should obtain a license before driving on the road.
开车上路前你要先有一个驾照。
I had my mechanic inspect the car before I bought it.
在购买之前，我让技师先检查了一遍这部车。
Please be here before six.
请在六点以前到达这里。
※The dictionary is the only place where success comes before work 成功先于工作的唯一地方是在字典里. 注:字典里字母S先于W." "
※Don't count your chicken before they are hatched. 不要过早乐观。
※Don't count your chickens before they are hatched. 蛋未孵，别先数鸡。
※Cast not your pearls before swine. 明珠莫投暗。
