现在完成时,ago,before的区别

1.ago表示“从现在起的若干时间以前”,意思是“距今……以前”,需和过去时或过去进行时连用.before泛指“从过去起的若干时间以前”,意思是 “距过去某时……以前”“与其……（毋宁）”,常和完成时连用,尤其在间接引语中,如：I visited him three days ago,but he had gone to Shanghai a week before．我三天前去访问他,但他已与那时的一周前到上海去了.
His parents died ten years ago．他父母十年前都去世了.
He said that his parents had died ten years before．他说他父母亲十年前都去世了.
I have never been there before．我以前从来没有去过那里.
2.如果不具体表明多少时间以前,只用before不用ago,意为“从前、以前”.before仍以副词的形式置于被修饰语后,常与完成时候过去时连用.如：
Have you seen this film before?
He asked me whether I had been to the Great Wall before．
3.表示在某一点时间或事件以前时,只用before不用ago,这种用法是将before当作介词或连词使用.ago不具有这一功能.如：
They will come back before six o’clock．
It is hoped that this will be finished before the year 1995．希望这件工作将在1995年以前完成.
另外,before在句中的含义较多.不少句子中,before虽然引导的也是一个时间状语,但是译成汉语时却不必译为“在……以前”.
a.如果before引导的从句动作发生得晚或慢,可译成“……才”.这是主句主语或是名词、代词,或用it作形式主语．
He had almost knocked me down before he saw me．他几乎把我撞倒才看见我.
It will be hours before he arrives．要过几个钟头他才回到达.
You must sow before you can reap．有播种才能有收成.
b.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.