并没有说一定用基本形式而不能用其他形式啊！

如：He can do it. He is doing it. He may be doing it. He has done it.He may have done it. He may have been doing it

助动词包括情态助动词和基本助动词。

情态助动词（简称情态动词）后面紧接的动词必须是不带to的不定式；不定式可以为不同的体。

基本助动词（简称助动词）有三个，be, have, do

助动词be只能后接二种形式；接现在分词构成进行体be doing;接过去分词构成被动语态be done.

助动词have只能后接过去分词构成完成体。have done

助动词do只能后接不带to的不定式构成否定、疑问、强调等语法形式。You do not want it. Do you do something? He does like music. 

其他连接方式都是错误的。