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.