在情态动词后面,动词要用原形 you may go, you should leave, he must die...
还有在助动词后面, I didn't do my homework.
I did eat three eggs.我的确吃了三个鸡蛋.
用to的时候,是动词的搭配,want to, have to, ought to
I want to swim. You have to go to school. You ought to give it up.
V+ing的时候很多,有动词词,现在分词.
I am reading a book. Let's go running. Let's go swimming.