before当主句用将来时，从句总是用现在时;如果before引导的从句谓语用的是过去时，则主句动词多用过去完成时，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。从句是过去完成时，主句是一般过去时。

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。

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)