before引导的从句不再用否定式的谓语，并且当before引导的从句位于主句之后，有时译成“就，才”。还要注意主句和从句之间的时间关系。当主句用将来时，从句总是用现在时；如果before引导的主句谓语用的是过去完成时，则从句动词多用一般过去时，这样以便体现动作发生的先后。After表示主句动作发生在从句动作之后。主句和 从句的动作的时间关系正好与before引导的从句相反。
例如：
It will be four days before they come back. 他们要过四天才能回来。
Einstein almost knocked me down before he saw me.爱因斯坦几乎把我撞倒才看到我。
My father had left for Canada just before the letter arrived.我父亲恰好在信到之前去加拿大了。
They had not been married four months before they were divorced. 他们结婚还不到四个月就离婚了。
After you think it over, please let me know what you decide.你仔细考虑过以后，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。（从句用过去完成时，主句用一般过去时）
