Prototypical conditional sentences in English are those of the form If X, then Y. The clause X is referred to as the antecedent (or protasis), while the clause Y is called the consequent (or apodosis). A conditional is understood as expressing its consequent under the temporary hypothetical assumption of its antecedent.

Conditional sentences can take numerous forms. The consequent can precede the if-clause and the word if itself may be omitted or replaced with a different complementizer. The consequent can be a declarative, an interrogative, or an imperative. Special tense morphology can be used to form a counterfactual conditional. Some linguists have argued that other superficially distinct grammatical structures such as wish reports have the same underlying structure as conditionals.

The consequent clause, expressing the consequence of the stated condition, is generally a main clause. It can be a declarative, interrogative, or imperative clause. It may appear before or after the condition clause:

• If I see him, I will tell him. (declarative sentence, condition first)
• I will tell him if I see him. (declarative sentence, condition second)
• If you saw him, would you tell him? (interrogative sentence, condition first)
• Would you tell him if you saw him? (interrogative sentence, condition second)
• If you see it, photograph it. (imperative sentence, condition first)
• Photograph it if you see it. (imperative sentence, condition second)

Like other languages, English uses past tense morphology to indicate that the speaker regards the antecedent as impossible or unlikely. This use of past tense is often referred to as fake past since it does not contribute its ordinary temporal meaning. Conditionals with fake past marking go by various names including counterfactuals, subjunctives, and X-marked conditionals.[1]

• Indicative: If it is raining in New York, then Mary is at home.
• Counterfactual: If it was raining in New York, then Mary would be at home.

In older dialects and more formal registers, the form were is often used instead of was. Counterfactuals of this sort are sometimes referred to as were'd up conditionals.

• Were'd up: If I were king, I could have you thrown in the dungeon.

The form were can also be used with an infinitive to form a future less vivid conditional.[3]

• Future Less Vivid: If I were to be king, I could have you thrown in the dungeon.

Counterfactuals can also use the pluperfect instead of the past tense.

• Conditional perfect: If you had called me, I would have come.