"How does it compares"

I understand it should be

How does it compare

because the do has stole the s, but what is the reasoning behind it? I don't want to make the same mistake twice, so I feel I need to understand the reasoning.

Because, as with can or should, do is a modal verb, and as such it is always followed by a verb in its basic form: basic form being the verb without the introducing to.

Compare can be transitive or intransitive.

Transitive, identifying differences:

John compared the two books and choose the larger

Intransitive, be comparable

The larger book just didn't compare with the little one