句子"all he had to do was……"为什么后面接的是动词原形,而不是doing或者to do

初中英语动词后面加动词原形或者后面加to do sth 或者加doing的所有动词 all you had to do was think a little harder ,was后面为什么加think的 动词原形后面接to do还是doing求高手解答 请问All you do now is to complete the form .中的 do 为什么是动词原形 stop……,后跟doing sth还是to do sth,to doing sth 或动词原形 be afraid of 后面为什么加to do ,而不是doing? practice doing 为什么practice的后面要跟doing 而不跟do 或者 to do would 后面是加to do 还是加动词原形? There are many advantages to 后面到底是接 do原形呢?还是doing? 情态动词后面+to do /doing had a hard time后面接to do还是doing? 为什么是be busy doing而不是be busy to do