An infinitive, is a verb that lacks finiteness, which basically means that either it lacks a subject or it is a subject. It is a type of verbal. There are two infinitives in English; a bare infinitive and a to-infinitive. Somewhat confusingly, either of these may be referred to simply as "the infinitive" without further specification.
|Bare infinitive||He made me do it!
We can do it!
|To infinitive||I want to go to the park
To complain would show disrespect.
Full modal verbs do not have infinitives. For example one cannot say *"to can"; one would have to employ a different way of achieving the same task, e.g. by using "to be able to" or "to be allowed to". See infinitive of a modal verb.