if引导的从句,只要不是虚拟语气,主句永远是将来时、祈使句、或情态动词的句子.从句可以是：一般现在时；现在完成时；现在进行时
1.If it rains tomorrow,we will stay at home.
2.If you have done your homework ,you can watch TV for a while.
3.If the baby is sleeping,don't make a noise.
希望能够帮到你!