The Infinitive is the base form of the verb:
be, have, walk…
Often the infinitive is introduced by the particle, to:
to be, to have, to walk…
Note that when the infinitive is on its own it’s sometimes known as the Bare Infinitive; when it’s with to then it’s sometimes known as the Full Infinitive.
Full Infinitive Usage/Infinitive Clauses
We use the full infinitive (the infinitive with…