1、主將從現(主句為一般將來時,從句為一般現在時);
eg:If it rains(從句) ,I will stay at home(主句).
=I will stay at home if it rains.
2、主句含有情態動詞 (must、may、can etc.)從句為一般現在時;
eg:If you drive too fast ,you may have an accident.
=You may have an accident if you drive too fast
3、主句為祈使句,從句要用一般現在時態;
eg:Dont play computer games if you dont finish your homework.