辨析once 和before

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