A conditional sentence connects a condition with a result. Most English conditionals use an if-clause for the condition and a main clause for the result. The verb form in each clause shows whether the situation is seen as general, possible, unlikely, or imagined. The choice of conditional type signals how real or hypothetical the situation is.

The zero conditional describes general truths or regular results. It uses the present simple in both the if-clause and the main clause. It does not express possibility or intention; it states what usually happens when the condition is met.

Rule
Use the present simple in both clauses to state a general truth.
The if-clause names a condition, and the main clause states the result.
Zero conditional does not refer to a specific future event.

The first conditional describes a real or likely possibility in the future. It uses the present simple in the if-clause and will plus the base verb in the main clause. It can also use can, may, or might in the main clause to show different degrees of certainty or permission.

Rule
Use the present simple in the if-clause and will plus base verb in the main clause for a likely future result.
The first conditional refers to situations the speaker sees as possible or probable.
Can, may, or might can replace will to adjust meaning in the main clause.

The second conditional describes hypothetical or unlikely situations in the present or future. It uses the past simple in the if-clause and would plus the base verb in the main clause. The time reference is often present or future, but the situation is not expected to be real.

Rule
Use the past simple in the if-clause and would plus base verb in the main clause for a hypothetical result.
The second conditional expresses improbability or imagination, not past events.
Could or might can replace would to show ability or possibility.

In formal English, were is used with all subjects in hypothetical if-clauses, especially with if I were and if he were. This is common in the second conditional when the condition is especially unreal or imaginary. In informal English, was is often used with I, he, she, and it, but were remains standard in formal writing.

Rule
Use if I were for a strongly hypothetical condition in formal English.
Were can be used with any subject in formal hypothetical if-clauses.
Was appears in informal speech but is less formal than were in this context.

The third conditional describes hypothetical situations in the past and their imagined results. It uses the past perfect in the if-clause and would have plus past participle in the main clause. It does not describe the actual past; it describes what could have happened under different conditions.

Rule
Use the past perfect in the if-clause and would have plus past participle in the main clause for past hypotheticals.
The third conditional refers to an unreal past, not to past facts.
Could have or might have can replace would have to show ability or possibility.

Mixed conditionals combine different time references in the if-clause and main clause. A common pattern is past condition with present result, using the past perfect in the if-clause and would plus base verb in the main clause. Another pattern is present condition with past result, but this is less frequent.

Rule
Use past perfect in the if-clause and would plus base verb in the main clause to show a past condition affecting the present.
Mixed conditionals link an unreal past to a present consequence or state.

If introduces a condition, while unless introduces an exception or negative condition. Unless means if not and is usually followed by the present simple in zero and first conditionals. The choice between if and unless changes the focus of the sentence but often leads to similar meanings.

Word/Phrase	Definition
if	If introduces a condition that may lead to a result.
unless	Unless introduces a negative condition meaning if not.

The if-clause and the main clause can switch places without changing the meaning. When the if-clause comes first, a comma usually separates the two clauses. When the main clause comes first, a comma is not usually needed.

Rule
Both orders are correct: if-clause first or main clause first.
Use a comma after the if-clause when it comes first.
Do not use a comma when the main clause comes first.

English conditionals use verb forms to signal whether a situation is general, possible, hypothetical, or imagined in the past. Zero conditional uses present simple for general truths. First conditional uses present simple and will for likely future events. Second conditional uses past simple and would for present or future hypotheticals. Third conditional uses past perfect and would have for past hypotheticals. Mixed conditionals combine these patterns to connect different times.