以一个动词开头时,什么时候是祈使句,什么时候是动名词作主语,什么时候加to表目的

动词原形开头的是祈使句如：Close the door.(肯定) 或Don't be late.（否定）
doing 形式作主语一般是在讲某件事情怎么样,后面会有谓语动词.如： Skydiving is my favorite.
Reading has become an important part in my life.
to do 表示目的（状语）,一般情况下后面会有主谓,并且可用why提问.如： To pass the exam, he stayed up late every day.对to pass the exam提问, Why did he stay up late every day?