几道语法题
问题描述：
几道语法题
You can arrive in Beijing earlier for the meeting ( )you don’t mind travelling by air.
为什么用provided 不是until
Government cannot operate effectively ( )it is free from such interference.
为什么用unless 不用if only
His father had promised to buy him a computer ( )he behaved himself.
为什么用provided 不用after
答
provided假设,如果 until 直到,在.之前 （主句动词要为延续才可以）
if only 要是.就好了（从句虚拟） 不是only if
用after的话 had promised 所表达的时间在前啊