如果在英语中,句子的开头是动词,那么这个动词什么时候用原型,什么时候用动名词,什么时候用不定式?

当要用动词相关的动作作主语时,动词要用+ing形式.
Smoking is bad for your health.
用动词原形的几情况
1.加助动词do,does或did 的肯定句变成否定句时,其后面的动词要用动词原形；
He didn’t work hard at English last term.
2.情态动词后面要用动词原形；
Must I be at home tonight?
3.be going to,will,shall 等构成的将来时态,其后面的动词要用动词原形；
There will be on a new interesting film at the cinema.
4.祈使句要用动词原形；Be careful,Lin tao!
5.let开头的祈使句,其后面的动词要用动词原形；
Let’s have a look at the pictures.
6.had better 后面要用动词原形；
you’d better put on more clothes.It’s very cold outside.
7.why not 之后要用动词原形；
Why not come a little earlier?= Why don’t you come a little earlier?
8.动词不定式to之后要用动词原形；
Please give me something nice to eat.
9.feel,hear,see,watch 等感官动词之后要用动词原形.如果表示这一动作正在进行,就用现在分词.
Sometimes we can hear a girl sing in the room.
I feel my left leg hurting badly.
表示目的时,一般用不定式 to do开头.