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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Use to + base verb for the standard infinitive form	🟦 使用 to + 基本动词表示标准不定式形式	I want to learn English.	🟩 我想学习英语。
Use a to infinitive to express purpose	🟦 使用带 to 的不定式来表达目的	She went outside to call her friend.	🟩 她走到外面去给她的朋友打电话。
After many adjectives, use a to infinitive to add meaning	🟦 在许多形容词后使用带 to 的不定式来增加意义	It is hard to focus today.	🟩 今天很难集中注意力。
Keep the infinitive in base form even with he or she	🟦 即使主语是 he 或 she，也保持不定式的基本形式	He needs to leave now.	🟩 他现在需要离开。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
After modal verbs, use a bare infinitive	🟦 情态动词之后，使用裸不定式	You must leave now.	🟩 你现在必须离开。
After do, does, did in questions and negatives, use a bare infinitive	🟦 在 questions 和 negatives 中，do、does、did 之后使用裸不定式	Did you call her?	🟩 你打电话给她了吗？
After make and let, use a bare infinitive	🟦 在 make 和 let 之后，使用裸不定式	They made me wait.	🟩 他们让我等待。
After see or hear, a bare infinitive can show the whole action	🟦 在 see 或 hear 之后，裸不定式可表示整个动作	I saw him cross the street.	🟩 我看到他穿过街道。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
An infinitive phrase can be the subject	🟦 不定式短语可以作主语	To travel alone takes courage.	🟩 单独旅行需要勇气。
Use it as a more natural subject with long infinitive phrases	🟦 对较长的不定式短语，作为更自然的主语使用它	It takes courage to travel alone.	🟩 单独旅行需要勇气。
An infinitive phrase can be the object of a verb	🟦 不定式短语可以作动词的宾语	We decided to stay home.	🟩 我们决定呆在家里。
An infinitive can be a complement after be	🟦 不定式在 be 之后可以作表语	My goal is to improve my pronunciation.	🟩 我的目标是提高我的发音。

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	🇨🇳 中文 词/短语	Definition	🇨🇳 中文 定义	Example	🇨🇳 中文 示例
want to	🟦 想要去	desire an action	🟩 表示一个动作的愿望	I want to rest.	🟩 我想休息。
plan to	🟦 打算要做某事	intend to do something	🟩 打算去做某事	They plan to move soon.	🟩 他们计划很快搬家。
decide to	🟦 决定要做某事	choose to do something	🟩 选择去做某事	She decided to call him.	🟩 她决定给他打电话。
try to	🟦 试着去做某事	make an effort to do something	🟩 努力去做某事	I try to eat healthy.	🟩 我努力吃得健康。
need to	🟦 需要去做某事	have to do something	🟩 必须去做某事	We need to leave early.	🟩 我们需要提前离开。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Use adjective + to infinitive to evaluate an action	🟦 使用形容词 + to 不定式来评估一个动作	This is easy to understand.	🟩 这很容易理解。
Use adjective + to infinitive for emotions or reactions	🟦 使用形容词 + to 不定式来表达情感或反应	I am glad to meet you.	🟩 我很高兴见到你。
Use too + adjective + to infinitive to show excess	🟦 使用 too + 形容词 + to 不定式 来表示过量	The coffee is too hot to drink.	🟩 咖啡太烫，喝不下去。
Use adjective + enough + to infinitive to show sufficiency	🟦 使用 形容词 + enough + to 不定式 来表示充足	She is old enough to drive.	🟩 她年龄足以开车。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Use to + verb to answer why	🟦 使用 to + 动词来回答“为什么”	He saved money to buy a laptop.	🟩 他存钱是为了买一台笔记本电脑。
Use in order to for extra clarity or formality	🟦 为了额外清晰或正式，使用 in order to	She spoke slowly in order to be clear.	🟩 她为了让人清晰而慢慢地说话。
Use so as to as a formal alternative	🟦 作为正式替代，使用 so as to	He left early so as to avoid traffic.	🟩 他提前离开以避免交通。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
An infinitive phrase can include objects and modifiers	🟦 不定式短语可以包含宾语和修饰语	She hopes to finish the report on time.	🟩 她希望按时完成报告。
Use for + noun or pronoun + to infinitive to show the subject of the infinitive	🟦 使用 for + 名词或代词 + to 不定式来表明不定式的主语	It is important for them to arrive early.	🟩 让他们尽早到达很重要。
This structure is common after adjectives like important and necessary	🟦 这种结构在形容词如 important 和 necessary 之后很常见	It is necessary for you to sign here.	🟩 你在这里签字是必要的。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
A split infinitive places an adverb between to and the verb	🟦 分裂不定式在 to 和动词之间放置副词	She decided to quickly respond.	🟩 她决定快速地回应。
Use splitting when it improves clarity or naturalness	🟦 当分裂能提高清晰度或自然度时使用	He wants to really understand the topic.	🟩 他想要真正理解这个话题。
Avoid splitting if your style guide requires it, but keep meaning clear	🟦 若你的风格指南要求避免分裂，请避免，但要确保意思清楚	He wants really to understand the topic.	🟩 他真心地要理解这个话题。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Form the perfect infinitive with to have + past participle	🟦 用 to have + 过去分词来形成完成时不定式	She seems to have forgotten.	🟩 她似乎忘记了。
Use it when the infinitive action is earlier than the main verb	🟦 当不定式动作早于主句动词时使用	He was happy to have finished early.	🟩 他很高兴提前完成。
Use it for past criticism or regret with should or ought	🟦 与 should 或 ought 一起用于对过去的批评或遗憾	You should have called me.	🟩 你应该打电话给我。

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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Form the passive infinitive with to be + past participle	🟦 用 to be + 过去分词来构成被动不定式	She wants to be promoted.	🟩 她想被提升。
Use it when the doer is not important or not known	🟦 当施动者不重要或未知时使用	The form needs to be signed today.	🟩 该表格今天需要被签署。
It can follow adjectives about expectations	🟦 它可以跟在表达期望的形容词之后	He is likely to be selected.	🟩 他很可能会被选中。