由before和after引导的时间状语从句
before引导的时间状语从句表示主句动作发生在从句动作之前.从句不用否定的谓语形式.从句位于主句之后时,before有时译成“才；就”.还要注意主句和从句的时间关系.当主句用将来时,从句总是用现在时；从句用过去时,则主句多用过去完成时,以此来体现动作发生的先后顺序.
after表示主句动作发生在从句动作之后,主句和从句的时间关系正好与before相反.如：
It will be four days before they come back.
他们要过四天才能回来.
My father had left for Canada just before the letter arrived.
我父亲恰好在信到达之前去加拿大了.
After you think it over,please let me know what is your decision.
你仔细考虑过之后,告诉我你是怎样决定的.
After we had finished the work,we went home.
完成工作之后,我们回家了.