An infinitive is the plain, or uninflected, form of a verb (go, run, fall, talk, dress, shout) and is generally preceded by the word to, which in this case is not a preposition but rather the sign of an infinitive.
It is time to go to school.
Let me show you the campus. [To is not used with the infinitive after certain verbs, including let, make, help, see, and hear.]
An infinitive is a verbal and may function as a noun, an adjective, or an adverb.
To pass is not the only objective, [noun]
These are the classes to take, [adjective]
He was too tired to study, [adverb]
The infinitive may reflect two tenses: the present and (with a helping verb) the present perfect.
to go [present tense]
to have gone [present perfect tense]
Do not use the present perfect tense when the present tense is sufficient.
Infinitives of transitive verbs can express both active and (with a helping verb) passive voice.
to hit [present tense, active voice]
to have hit [present perfect tense, active voice]
to be hit [present tense, passive voice]
to have been hit [present perfect tense, passive voice]
A split infinitive occurs when an adverb or an adverb phrase is placed between the sign of the infinitive, to, and the infinitive itself. A modifier, especially a long one, between the two words can often be awkward; in most cases, it is preferable not to split an infinitive.
Sometimes, however, splitting an infinitive is necessary to prevent awkwardness or ambiguity. Compare the following sentences.
He opened the envelope unexpectedly to find the missing papers. [Unexpectedly seems to modify opened rather than find.]
He opened the envelope to find unexpectedly the missing papers. [This sentence is awkward.]
He opened the envelope to unexpectedly find the missing papers. [Splitting the infinitive is the least awkward way to modify find.]
SPECIAL PROBLEMS WITH INFINITIVES
Infinitives do not show past tense, number, or person. Do not add ed or s to them.
The particle to is usually omitted after the helping verbs can, could, will, would, shall, should, and must (but not after need, have, and ought); after sensory verbs (such as see, hear, watch, and listen); and after a few other verbs such as help, make, and let.
Frequently, I watch the children to play in the neighborhood park. The sound of their voices makes me to remember my childhood in Vietnam.
To indicate purpose, use an infinitive (rather than a gerund) as a complement ("I enrolled in this class to meet people").
With four verbs (stop, remember, forget, regret) that can be followed by either an infinitive or a gerund as a complement, the meaning changes according to which is used. After stop, an infinitive indicates a purpose, and a gerund indicates an activity that ceases.
On my way home, I stopped to talk to my friend. I stopped talking to my friend after she insulted me.
After remember, forget, or regret, an infinitive refers to action after the time of the main verb, and a gerund refers to action before the time of the main verb.
I'll never forget to hand in that paper. I'll never forget handing in that paper.
(For a list of verbs that are used only with gerunds as complements, see gerunds.)