(2)如果主句含有must,may,can等情态动词,从句也要用一般现在时。 If you drive too fast,you may have an accident.如果你把车开得太快,你可能会出事故。 You must stop if the traffic light is red.如果红灯亮了,你必须停下来。 (3)如果主句是祈使句,从句也要用一般现在时。 Don't go and play football if you don't finish your homework.如果你没完成家庭作业,就不要去踢足球。 Please put up your hands if you have any question.如果你们有问题的话,请举手。