before引导的从句的时态比主句早一个时态，after引导的从句时态比主句晚一个时态。
时间状语从句：用表示时间的连词连接一个句子作状语，这样的主从复合句就是时间状语从句。
before有“才，就”的意思，当主句用将来时，从句总是用现在时。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与before正好相反，从句用过去完成时，主句用一般过去时，例如：
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。
before，根据具体语境还可理解为“还没来得及/还没有……就……，趁着还没……就……，不知不觉就……，……才……”等。如：
I had not sat long before he came back. 我没坐多久他就回来了。
He almost knocked me down before he saw me. 他几乎把我撞倒才看到我。
Before I could get in a word, he measured me. 我还来得及插话，他就给我量好了尺寸。
I’ll water the flowers before it gets dark. 我要趁着天还没黑把花浇好水。
Be a pupil before you become a teacher. 先做学生，后做先生。
Time passed quickly and three months went by before she knew it. 时间过得飞快，她不知不觉就在那里度过了三个月
I will die before I will betray my country. 我宁可死也不会背叛我的国家。
参考资料
百度百科：https://baike.baidu.com/item/%E6%97%B6%E9%97%B4%E7%8A%B6%E8%AF%AD%E4%BB%8E%E5%8F%A5/7926760#4_1