1、主将从现(主句为一般将来时，从句为一般现在时)。
eg:If it rains(从句) ,I will stay at home(主句)。
=I will stay at home if it rains.
2、主句含有情态动词 (must、may、can etc.)从句为一般现在时。
eg:If you drive too fast ,you may have an accident.
=You may have an accident if you drive too fast.
3、主句为祈使句，从句要用一般现在时态。
eg:Dont play computer games if you dont finish your homework.
if一共引导三种从句：一是条件状语从句，二是宾语从句，三是引导非真实条件句。
1引导的三种从句
一是条件状语从句，译为“如果”。
He will come if you invite him. 如果你请他，他会来的。
二是宾语从句，译为“是否”。
I wonder if she is ill. 不知她是否病了。
三是引导非真实条件句，使用虚拟语气，译为“假如,要是”。
If I were you, I would never do that.
如果我是你，我决不做那件事。
2拓展
一、由when , as soon as , not until 等引导的时间状语从句也是遵循“主将从现”原则。
如：I will tell him about it as soon as he comes back.
他一回来我就告诉他这件事。
二、if 也可引导宾语从句，意思是”是否“
如： I don't know if he will come tomorrow.
我不知道他明天是否回来。
