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、主甜入艺阝句含有情态动词 （must、may、can etc.）从句为一般现在时；eg:If you drive too fast ,you m锾攒揉敫ay 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:Don't play computer games if you don't finish your homework.
4、备注：1、If 于句首时中间用逗号隔开,主句后不用逗号.2、if 可用unless(除非...) 替换.unless= if...not...3、注意虚拟语气 主句和从句都为一般过去时
