三句口诀：“主现从任、主过从过、主将从现。”
意思是说：
主句是现在时的话，从句随便你用什么时态。
比如：I‘ll tell her the good news when she comes back.
Don’t laugh at me when I make a mistake.(祈使句)
You should be quiet when you are in the library.(含情态动词)
主句是过去式的话，从句用四种过去时态中的一种（过去一般、过去进行、过去完成、过去将来）
比如：When he entered the room, I stopped eating.
When he entered the room, I was eating.
When he entered the room, I had been eating for five minuted.
When he entered the room, I would have finished eating.这个不太恰当，但你能理解就行。
主句是将来时的话，从句用现在时。
I will be a nurse when I grow up.
另外，since 引导时间状语从句时，其主句通常要用现在完成时，有时也用现在完成进行时。如：
She has had another baby since I saw her last. 从上次我见到她以来，她又生了一个孩子。
He has grown an inch since I saw him. 我上次见到他以后他已长高了一英寸。
注：当主句为“It + be + 一段时间”时，则通常用一般现在时代替现在完成时。如：
It’s more than three years now since I saw her last. 我们已有三年多没见面了。
一些特定句式的时态：
由no sooner. . . than, hardly [scarcely]…when引出的时间状语从句，其主句通常用过去完成时，从句通常用一般过去时。如：
Hardly had he arrived when she started complaining. 他刚到她就抱怨起来。
No sooner had he gone to sleep than the telephone rang. 他刚睡着电话铃就响了。
We had no sooner set out than a thunderstorm broke. 我们刚出发就下起一阵雷暴。
No sooner had she agreed to marry him than she started to have terrible doubts. 她刚同意嫁给他她就开始产生可怕的疑虑。
When you have finished your homework, I will give you 10 dollars.做完作业，我给你十美元作为奖励。
When he has come back, I have finished my homework.你到家的时候，我肯定已经完成家庭作业了。
不懂问我，希望对您有帮助，南无地藏菩萨！