An infinitive is the base form of a verb. In English it often appears with to, as in to eat or to study, but it can also appear without to after certain verbs. Infinitives can act like nouns, adjectives, or adverbs depending on where they appear in the sentence. They often express purpose, intention, or an unrealized action rather than a completed event.

A to infinitive is formed with to + base verb: to go, to see, to understand. It is common after many verbs, after adjectives, and to express purpose. Do not change the verb form for tense or subject agreement inside an infinitive: we say to work, not to works or to worked. The time meaning usually comes from the main verb, not the infinitive itself.

Rule	Example
Use to + base verb for the standard infinitive form	I want to learn English.
Use a to infinitive to express purpose	She went outside to call her friend.
After many adjectives, use a to infinitive to add meaning	It is hard to focus today.
Keep the infinitive in base form even with he or she	He needs to leave now.

A bare infinitive is the base verb without to: go, eat, help. It is used after auxiliary verbs like can and should, and after certain verbs such as make, let, and help in many structures. It is also used after verbs of perception like see and hear in specific patterns. Bare infinitives often feel more direct and immediate than to infinitives.

Rule	Example
After modal verbs, use a bare infinitive	You must leave now.
After do, does, did in questions and negatives, use a bare infinitive	Did you call her?
After make and let, use a bare infinitive	They made me wait.
After see or hear, a bare infinitive can show the whole action	I saw him cross the street.

An infinitive phrase can function as a noun, meaning it can be the subject, object, or complement of a sentence. When used as a subject, it often sounds formal or abstract, and it is common to use an anticipatory it structure instead. As an object, it commonly follows verbs that describe wants, plans, decisions, or attempts. The infinitive phrase can include its own objects and modifiers, forming a full noun-like unit.

Rule	Example
An infinitive phrase can be the subject	To travel alone takes courage.
Use it as a more natural subject with long infinitive phrases	It takes courage to travel alone.
An infinitive phrase can be the object of a verb	We decided to stay home.
An infinitive can be a complement after be	My goal is to improve my pronunciation.

Many verbs are followed by infinitives, especially verbs about intention, choice, or effort. Some verbs take a to infinitive, while others take a bare infinitive, and some can take either with a change in meaning. Learning the pattern is important because it affects correctness and naturalness. When an infinitive follows a verb, the infinitive usually describes the next action or the purpose of the first action.

Word/Phrase	Definition	Example
want to	desire an action	I want to rest.
plan to	intend to do something	They plan to move soon.
decide to	choose to do something	She decided to call him.
try to	make an effort to do something	I try to eat healthy.
need to	have to do something	We need to leave early.

Infinitives commonly follow adjectives to explain feelings, difficulty, readiness, or suitability. This pattern often evaluates an action: easy to learn, hard to explain, ready to go. It can also describe someone’s reaction: happy to help, surprised to hear. The infinitive supplies the action that completes the meaning of the adjective.

Rule	Example
Use adjective + to infinitive to evaluate an action	This is easy to understand.
Use adjective + to infinitive for emotions or reactions	I am glad to meet you.
Use too + adjective + to infinitive to show excess	The coffee is too hot to drink.
Use adjective + enough + to infinitive to show sufficiency	She is old enough to drive.

An infinitive can explain why someone does something, giving a purpose or goal. This use is especially common with movement and actions: went to, came to, called to, studied to. Purpose infinitives are usually introduced with to, and they answer the question why. For emphasis or clarity, you can also use in order to, especially in formal writing or when the meaning could be confusing.

Rule	Example
Use to + verb to answer why	He saved money to buy a laptop.
Use in order to for extra clarity or formality	She spoke slowly in order to be clear.
Use so as to as a formal alternative	He left early so as to avoid traffic.

An infinitive can expand into an infinitive phrase that includes objects, complements, and modifiers. The whole phrase functions as a single unit in the sentence, so word order inside the phrase matters. You can also use an infinitive with its own subject, usually introduced with for: for him to understand, for us to leave. These longer phrases often appear after adjectives and nouns.

Rule	Example
An infinitive phrase can include objects and modifiers	She hopes to finish the report on time.
Use for + noun or pronoun + to infinitive to show the subject of the infinitive	It is important for them to arrive early.
This structure is common after adjectives like important and necessary	It is necessary for you to sign here.

A split infinitive places a word, usually an adverb, between to and the base verb, as in to really understand. In modern English this is widely accepted and often sounds natural, especially when the adverb needs to clearly modify the verb. Some formal styles prefer to avoid splitting, but forcing the adverb elsewhere can change meaning or make the sentence awkward. Choose the placement that is clearest and most natural for your context.

Rule	Example
A split infinitive places an adverb between to and the verb	She decided to quickly respond.
Use splitting when it improves clarity or naturalness	He wants to really understand the topic.
Avoid splitting if your style guide requires it, but keep meaning clear	He wants really to understand the topic.

A perfect infinitive uses to have + past participle: to have eaten, to have finished. It is used when the infinitive action happens earlier than the main verb time, often to express regret, criticism, or inference about the past. It frequently appears after verbs like seem, appear, claim, and after modal meanings such as should. The perfect infinitive helps show relative time without changing the main verb tense.

Rule	Example
Form the perfect infinitive with to have + past participle	She seems to have forgotten.
Use it when the infinitive action is earlier than the main verb	He was happy to have finished early.
Use it for past criticism or regret with should or ought	You should have called me.

A passive infinitive focuses on the receiver of the action rather than the doer. It is formed with to be + past participle, as in to be invited or to be chosen. Use it when the agent is unknown, unimportant, or already understood. Passive infinitives are common after verbs and adjectives that refer to expectations, plans, or requirements.

Rule	Example
Form the passive infinitive with to be + past participle	She wants to be promoted.
Use it when the doer is not important or not known	The form needs to be signed today.
It can follow adjectives about expectations	He is likely to be selected.