[Grammar] She + Can + Do...........Why?

In a positive sentence we use:

He does his homework everyday.

Why with the modal can does it then change to DO with he & She?

He can do his homework everyday if he wants to.

Many thanks

[David L.]

The infinitive, the basic form of the verb, is 'to do'. When verbs are used in isolation, they are inflected:
I do
He does
They do

When we include a modal verb, the form is, modal +infinitive
I can do
He can do
they can do
we can do