辨析once和beforeonce和before都有‘以前,曾经’的词义,都是副词.它们有什么区别?比如：1》Isawhimbefore,butIcan'trememberhisname.2>WeoncelivedinBeijinng.3>Before,withbothmanandwifeworking,therewasnotal

Question

辨析once 和before
once和before 都有‘以前,曾经’的词义,都是副词.它们有什么区别?
比如：1》I saw him before,but I can't remember his name.
2> We once lived in Beijinng.
3> Before,with both man and wife working,there was not always this kind of problem.
4>Once he was interested in a computer.
上述4句里的once 和before 是否都可以互换?
我觉得,主要是位置区别和时态区别.before 要么置句首,要么句尾,不会置实义动词前.once 要么置句首,要么置实义动词前,不会置句尾.
还有,before 更常与过去完成时连用,（Haven't I seen you?/ we had met on the Saturday before),而 once 更多与一般过去时连用.
在词义上会有什么区别吗?高手有什么补充吗

Answer 1

1 Not once
放句首时,后接倒装语序
Not once have you done as I asked
你一直也没有照我的要求去做
2 作连词时,所引导的从距离不用将来时
Once you show any fear,he will attack you.
你一显示害怕,他就会袭击你
3 作连词时,常省略主语和be
Once (it is)repaired,the machine can be put into use again.
机器一经修理就可以使用了
4 这个是最重要的
作adv(副词)时,不用于将来时,放v.前为"曾经",放句尾为"一次"
那部电影我看过一次
I saw that film once.
你的第一个问题:不可互换
I saw him before我以前见过他
I saw him once我看过他一次
We once lived in Beijing,我们曾经在北京住过(在动词前的once,第四点提到)
3 4根据意思,还是不可互换
第二个问题:你说的是对的