To form infinitive forms of verbs in English, the construction uses the word “to” plus the verb. For example:
to love
to go
to speak
Infinitives may be in the present tense such as:
to be
to see
to throw
or they may be in the perfect tense which is formed with the word “to have” plus the past participle of the verb such as:
to have been
to have seen
to have spoken
Infinitives may also be in the passive voice such as:
to be liked
to be seen
to be promoted
Often, the infinitive is paired with another verb in a sentence such as:
Sarah likes to go to the city.
Robert is able to see the stage.
Sometimes an infinitive may function as the subject of a sentence as in:
To walk is healthy exercise.
'To err is human...'