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 protasis or condition, and the apodosis or result.
- If it rains [protasis], the picnic will be cancelled [apodosis].
The apodosis is syntactically the main clause, and the protasis is a subordinate clause. It is primarily the properties of the protasis (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 protasis. The terms "realis" and "irrealis" broadly correspond to the notions of realis and irrealis modality.
[edit] Realis conditions
The verb in the protasis is in the past tense (with a past tense interpretation) or in the present tense (with a present or future tense interpretation). The apodosis can be in the past, present, or future.
This class includes universal statements (both clauses in the present, or both clauses in the past):
- If water is heated to 100 degrees, it boils.
- If it's cloudy in the morning, I take my umbrella.
- In the Dark Ages, if someone had the plague, their entire family was locked away.
More generally, the protasis expresses a hypothetical condition that is potentially true, but not yet verified. The past tense refers to a state or event in the past, and the result can be in the past, present, or future.
- 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 protasis in the present tense refers to a current state or event (which may or may not be true) or a state or event that could be verified in the future. The result can be in the past, present, or future.
- If it's raining here now, it was raining on the West Coast this morning.
- If it's raining now, 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.
Note that future tense forms are not used in the protasis in English: *If it will rain this afternoon, …
[edit] Irrealis conditions
In these constructions, the protasis expresses a condition that is known to be false, or presented as unlikely. The apodosis contains a conditional verb form consisting of would (or could, should, might) plus an infinitival main verb.
To refer to a current state or event that is known to be false, 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.
To refer to contrary-to-fact past events, the pluperfect (or past perfect subjunctive) is used in the protasis.
- 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 protasis 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 regularly use would have in the protasis for past reference: If you would've told me, we could've done something about it.
Should can appear in the protasis 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.
