It isn't a verb. It's an adjective.
"A sure thing." Sure is the adjective modifying the noun thing.
"I'm sure you are right." Sure is the adjective modifying I.
'To be sure' is a verb 'am sure' is the 'I' part
Neither, "sure" is not a verb."sure" can be either an adjective or an adverb.
A linking verb.
A linking verb
It is an irregular verb.
The verb appear can be a linking verb or an action verb.
An auxiliary verb
the infinative
Is becomes a verb
It is a regular verb
a regular verb.
Be is a state of being verb.