Other than its usual forms like 'to see', 'to go', the infinitives are also used in different forms.
Look at the following sentences:
a. Out teacher makes us work hard.
b. I heard a man cry bitterly.
In these sentences, 'work' and 'cry' are infinitives used without 'to'.
Look at the following sentences:
a. Many people are reported to have died in the accident.
b. He is said to have completed the work.
In these sentences 'to have died' and 'to have completed' are infinitives used in Perfect Infinitive forms.
Look at the following sentences:
a. Ramesh seems to be studying late hours at night.
b. He appears to have been working on this machine for years.
In these sentences, 'to be studying', 'to have been working' are infinitives used in the present continuous infinitive and Present Perfect continuous infinitive forms.
A Non-Finite Verb is different in its function as a verb from that of a Finite Verb in respect of its not having a Subject. Though a non-finite verb retains some of the characteristic functions of the verb like having an object, a complement or an adverb, it functions in a sentence as Noun, Adjective or Adverb. Infinitive, Participle and Gerund are all Non-Finite forms of a Verb.
Also See: Infinitives