Conditional sentence

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In grammar, conditional sentences are sentences discussing factual implications or hypothetical situations and their consequences. Languages use a variety of conditional constructions and verb forms (such as the conditional mood) to form these kinds of sentences.

Full conditional sentences contain two clauses: the condition or protasis, and the result or apodosis.

If it rains [condition], (then) the picnic will be cancelled [result].

Syntactically, the result is the main clause, and the condition is a subordinate clause. It is primarily the properties of the protasis (condition) (tense and degree of factualness), however, that determine the properties of the entire sentence.

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[edit] Conditional sentences in Latin

Conditional sentences in Latin are traditionally classified into three categories, based on grammatical structure.

  • simple conditions (factual or logical implications)
    • present tense [present indicative ? indicative]
    • past tense [perfect/imperfect indicative ? indicative]
  • future conditions
    • "future more vivid" [future indicative ? future indicative]
    • "future less vivid" [present subjunctive ? present subjunctive]
  • contrafactual conditions
    • "present contrary-to-fact" [imperfect subjunctive ? imperfect subjunctive]
    • "past contrary-to-fact" [pluperfect subjunctive ? pluperfect subjunctive]

[edit] Conditional sentences in English

English conditional sentences can be divided into two broad classes, depending on the form of the verb in the condition (protasis). The terms "realis" and "irrealis" broadly correspond to the notions of realis and irrealis modality.

[edit] Realis conditions

In these constructions, the condition clause expresses a condition the truth of which is unverified. The verb in the condition clause is in the past tense (with a past tense interpretation) or in the present tense (with a present or future tense interpretation). The result clause can be in the past, present, or future. Generally, conditional sentences of this group are in two groups, the "zero" conditional and the potential or indicative conditional. This class includes universal statements (both clauses in the present, or both clauses in the past) and predictions.

The "zero" conditional is formed with both clauses in the present tense. This construction is similar across many languages. It is used to express a certainty, a universal statement, a law of science, etc.:

If you heat water to 100 degrees celsius, it boils.
If you don't eat for a long time, you become hungry.
If the sea is stormy, the waves are high.

It is different from true conditionals because the introductory "if" can be replaced by "when" or "whenever" (e.g., "When you heat water..."), which cannot be done for true conditionals.

The potential or indicative conditional (sometimes referred to as a "first" conditional) is used more generally to express a hypothetical condition that is potentially true, but not yet verified. The conditional clause is in the present or past tense and refers to a state or event in the past. The result can be in the past, present, or future. Some examples with the condition clause in the past tense:

If she took that flight yesterday, she arrived at 10pm.
If she took that flight yesterday, she is somewhere in town today.
If she took that flight yesterday, we'll see her tomorrow.

A condition clause (protasis) in the present tense refers to a future event, a current event which may be true or untrue, or an event which could be verified in the future. The result can be in the past, present, or future:

If it's raining here now, then it was raining on the West Coast this morning.
If it's raining now, then your laundry is getting wet.
If it's raining now, there will be mushrooms to pick next week.
If it rains this afternoon, then yesterday's weather forecast was wrong.
If it rains this afternoon, your garden party is doomed.
If it rains this afternoon, everybody will stay home.
If I become President, I'll lower taxes.

Certain modal auxiliary verbs (mainly will, may, might, and could) are not used in the condition clause (protasis) in English:

*If it will rain this afternoon, …
*If it may have rained yesterday, …

In colloquial English, the imperative is sometimes used to form a conditional sentence: e.g. "go eastwards a mile and you'll see it" means "if you go eastwards a mile, you will see it".

[edit] Irrealis conditions

In these constructions, the condition clause expresses a condition that is known to be false, or presented as unlikely. The result clause contains a conditional verb form consisting of would (or could, should, might) plus an infinitival main verb.

The contrary-to-fact present conditional (sometimes referred to as the "second" conditional) is used to refer to a current state or event that is known to be false or improbable. The past subjunctive (or in colloquial English, simply the past tense) must be used:

If she were [colloq. was] at work today, she would know how to deal with this client.
If I were [colloq. was] king, I could have you thrown in the dungeon.

The same structure can be used to refer to a future state or event:

If I won the lottery, I would buy a car.
If he said that to me, I would run away.

In many cases, when referring to future events, the difference between a realis and irrealis conditional is very slight:

(realis) If you leave now, you can still catch your train.
(irrealis) If you left now, you could still catch your train.

The contrary-to-fact past conditional (sometime referred to as the "third" conditional) is used to refer to contrary-to-fact past events. The pluperfect (or past perfect) is used in the condition clause.

If you had called me, I would have come.
If you had done your job properly, we wouldn't be in this mess now.

Note that would-conditional forms are not usually used in the condition clause in English: *If you would leave now, you would be on time. There are exceptions, however: If you would listen to me once in a while, you might learn something. Some varieties of English regularly use would have in the protasis for past reference, although this is considered non-standard: If you would've told me, we could've done something about it.

Should can appear in the condition clause to refer to a future event presented as possible, but unlikely, undesirable, or otherwise "remote": If I should die before I wake, …, If you should ever find yourself in such a situation, …

[edit] The semantics of conditional sentences

The material conditional operator used in logic (i.e. "p ? q") is sometimes read aloud in the form of a conditional sentence (i.e. "if p, then q"), but the definition of this operator does not correspond to the intuitive interpretation of conditional statements in natural language. Modelling the meaning of real conditional statements requires the definition of an indicative conditional, and contrary-to-fact statements require a counterfactual conditional operator, formalized in modal logic.

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