Conditionals explain what happens if a condition is true or happens. They are built from two parts: the if-clause, which states the condition, and the main clause, which states the result. English uses different conditional patterns to show whether the condition is general, likely, unlikely, or impossible. Choosing the right pattern depends on time reference and how real the situation is.

A conditional sentence can start with the if-clause or the main clause. If the if-clause comes first, it is usually followed by a comma. If the main clause comes first, a comma is usually not used. The meaning stays the same; the order mainly affects emphasis and style.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
If-clause first uses a comma	🧩 如果从句在前，使用逗号	If it rains, we will stay inside.	🧩 如果下雨，我们就待在室内。
Main clause first usually has no comma	🧩 主句在前通常不带逗号	We will stay inside if it rains.	🧩 如果下雨，我们就待在室内。
Either order is acceptable	🧩 两种顺序均可接受	If you call, I can help.	🧩 如果你打电话，我可以帮忙。

Use the zero conditional for general truths, scientific facts, and habits where the result always follows the condition. Both clauses use the present simple because the sentence describes a timeless relationship, not a specific future event. This conditional often sounds like a rule or instruction.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Form: If + present simple, present simple	🧩 形式：If + 现在时，现在时	If you heat ice, it melts.	🧩 If you heat ice, it melts.
Use for facts and routines	🧩 用于事实和常规	If I drink coffee late, I sleep badly.	🧩 If I drink coffee late, I sleep badly.
Also common for rules and instructions	🧩 也常用于规则和指示	If the light is red, you stop.	🧩 If the light is red, you stop.

Use the first conditional for real and possible situations in the future and their likely results. The if-clause uses the present simple, and the main clause usually uses will plus the base verb. This pattern talks about a future consequence that depends on a condition that may happen.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Form: If + present simple, will + base verb	🧩 形式：If + 现在时，will + 基本动词	If it rains, we will cancel the picnic.	🧩 If it rains, we will cancel the picnic.
Use for likely future outcomes	🧩 用于可能的未来结果	If you study, you will pass.	🧩 If you study, you will pass.
Main clause can use may or might for weaker certainty	🧩 主句可以使用 may 或 might 表示较弱的确定性	If she calls, I might answer.	🧩 If she calls, I might answer.

Use the second conditional for unreal or unlikely situations in the present or future. The if-clause uses the past simple, but it refers to the present or future, not the past. The main clause uses would plus the base verb to show the result is hypothetical. This conditional is common for imagining different realities and giving advice politely.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Form: If + past simple, would + base verb	🧩 形式：If + 过去时，would + 基本动词	If I won the lottery, I would travel.	🧩 If I won the lottery, I would travel.
Past simple here signals unreality, not past time	🧩 这里的过去时表示非真实，而非过去时间	If he had more time, he would exercise.	🧩 If he had more time, he would exercise.
Use were for all subjects in formal style	🧩 在正式风格中对所有主语都用 were	If I were you, I would apologize.	🧩 If I were you, I would apologize.

Use the third conditional for unreal past situations: conditions that did not happen and results that did not happen. The if-clause uses past perfect, and the main clause uses would have plus the past participle. This conditional is common for regret, criticism, and imagining alternative past outcomes.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Form: If + past perfect, would have + past participle	🧩 形式：If + 过去完成时，would have + 过去分词	If we had left earlier, we would have arrived on time.	🧩 If we had left earlier, we would have arrived on time.
Use to express regret or missed opportunities	🧩 用于表达遗憾或错失的机会	If I had studied, I would have passed.	🧩 If I had studied, I would have passed.
Use for past criticism and evaluation	🧩 用于对过去的批评和评估	If you had listened, you would have understood.	🧩 If you had listened, you would have understood.

Mixed conditionals combine different time references in the condition and the result. A common pattern is a past condition with a present result, showing that something in the past still affects the present. Another pattern is a present condition with a past result, showing an ongoing state causing a past outcome. The tense choice reflects the time of each clause, not a fixed conditional type.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Past condition, present result: If + past perfect, would + base verb	🧩 过去条件，现时结果：If + 过去完成时，would + 基本动词	If I had taken the job, I would live in London now.	🧩 If I had taken the job, I would live in London now.
Present condition, past result: If + past simple, would have + past participle	🧩 现在条件，过去结果：If + 过去时，would have + 过去分词	If she were more careful, she would have avoided the mistake.	🧩 If she were more careful, she would have avoided the mistake.

The result clause does not always use will or would. Modals change meaning: can for ability, may or might for possibility, should for advice, and must for strong necessity. You can also use an imperative in the main clause to give instructions. The if-clause tense still follows the conditional type, while the modal shows attitude and strength.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
can for ability or permission	🧩 can 表示能力或许可	If you finish early, you can leave.	🧩 If you finish early, you can leave.
may or might for possibility	🧩 may 或 might 表示可能性	If he calls, I might be busy.	🧩 If he calls, I might be busy.
should for advice	🧩 should 表示建议	If you feel sick, you should rest.	🧩 If you feel sick, you should rest.
Imperative for instructions	🧩 用于指令的祈使句	If you see Anna, tell her to call me.	🧩 If you see Anna, tell her to call me.

Unless means if not and introduces the only condition that prevents the result. It is used in place of an if-clause with a negative meaning, but you usually keep the verb positive after unless. It commonly appears in rules, warnings, and agreements. The tense in the unless-clause follows the same logic as an if-clause.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
Unless = if not	🧩 Unless = 如果不	You will fail unless you study.	🧩 You will fail unless you study.
Usually avoid double negatives with unless	🧩 通常不要在 unless 中使用双重否定	I will go unless it rains.	🧩 I will go unless it rains.
Common in warnings and conditions	🧩 常见于警告和条件	Unless you pay today, we will cancel the order.	🧩 Unless you pay today, we will cancel the order.

English often uses alternatives to if to express conditions with different meanings and levels of formality. Provided that and as long as emphasize a strict requirement. In case means as preparation for a possible problem. Even if means the result is true despite the condition. These forms help you express precise conditional relationships beyond basic if.

Rule	🇨🇳 中文 Rule	Example	🇨🇳 中文 Example
provided that or providing that for strict conditions	🧩 provided that 或 providing that（用于严格条件）	You can borrow my car provided that you drive carefully.	🧩 You can borrow my car provided that you drive carefully.
as long as for requirements or limits	🧩 as long as 表示要求或限制	I will help as long as you are honest.	🧩 I will help as long as you are honest.
in case for precaution	🧩 in case 表示预防	Take a jacket in case it gets cold.	🧩 Take a jacket in case it gets cold.
even if for contrast	🧩 even if 用于对比/让步	Even if it rains, we will go.	🧩 Even if it rains, we will go.

Choosing a conditional is mainly about reality and time. Zero conditional states general cause and effect. First conditional predicts a likely future result. Second conditional imagines an unreal or unlikely present or future. Third conditional imagines a different past. Mixed conditionals link different times to show lasting consequences or background causes.