试用这角度来理解
never before 是‘从来未曾’
He has never been to Beijing before.
这句子是‘他从来未曾到过北京’
‘这次是他首次来到北京’
yet的意思有点著重於‘这一刻尚未’
如果要表达的意思是
‘他还未到达北京’(这里没有指明他以前曾否到过北京)
则会用 yet (此刻来说尚未0
He has not arrived at Beijing yet.
又例如
我还未做完家课
那便要用yet 而不用before了
I haven't finished my homework yet.