Conditional sentences are sentences expressing factual implications, or hypothetical situations and their consequences. They are so called because the validity of the main clause of the sentence is conditional on the existence of certain circumstances, which may be expressed in a dependent clause or may be understood from the context.
A full conditional sentence (one which expresses the condition as well as its consequences) therefore contains two clauses: the dependent clause expressing the condition, called the protasis; and the main clause expressing the consequence, called the apodosis. An example of such a sentence (in English) is the following:
- If it rains, the picnic will be cancelled.
Here the condition is expressed by the clause if it rains, this being the protasis, while the consequence is expressed by the picnic will be cancelled, this being the apodosis. (The protasis may either precede or follow the apodosis; it is equally possible to say "The picnic will be cancelled if it rains".) In terms of logic, the protasis corresponds to the antecedent, and the apodosis to the consequent.
Languages use a variety of grammatical forms and constructions in conditional sentences. The forms of verbs used in the protasis and apodosis are often subject to particular rules as regards their tense and mood. Many languages have a specialized type of verb form called the conditional mood – broadly equivalent in meaning to the English "would (do something)" – for use in some types of conditional sentence.
Types of conditional sentence 
There are various ways of classifying conditional sentences. One distinction is between those that state an implication between facts, and those that set up and refer to a hypothetical situation. There is also the distinction between conditionals that are considered factual or predictive, and those that are considered counterfactual or speculative (referring to a situation that did not or does not really exist).
Implicative and predictive 
A conditional sentence expressing an implication (also called a factual conditional sentence) essentially states that if one fact holds, then so does another. (If the sentence is not a declarative sentence, then the consequence may be expressed as an order or a question rather than a statement.) The facts are usually stated in whatever grammatical tense is appropriate to them; there are not normally special tense or mood patterns for this type of conditional sentence. Such sentences may be used to express a certainty, a universal statement, a law of science, etc. (in these cases if may often be replaced by when):
- If you heat water to 100 degrees, it boils.
- If the sea is stormy, the waves are high.
They can also be used for logical deductions about particular circumstances (which can be in various mixtures of past, present and 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 be picked next week.
- If he locked the door, then Kitty is trapped inside.
A predictive conditional sentence concerns a situation dependent on a hypothetical (but entirely possible) future event. The consequence is normally also a statement about the future, although it may also be a consequent statement about present or past time (or a question or order).
- If I become President, I'll lower taxes.
- If it rains this afternoon, everybody will stay home.
- If it rains this afternoon, then yesterday's weather forecast was wrong.
- If it rains this afternoon, your garden party is doomed.
- What will you do if he invites you?
- If you see them, shoot!
In a counterfactual or speculative conditional sentence, a situation is described as dependent on a condition that is known to be false, or presented as unlikely. The time frame of the hypothetical situation may be past, present or future, and the time frame of the condition does not always correspond to that of the consequence. For example:
- If I were king, I could have you thrown in the dungeon.
- If I won the lottery, I would buy a car.
- If he said that to me, I would run away.
- If you had called me, I would have come.
- If you had done your job properly, we wouldn't be in this mess now.
The difference in meaning between a "counterfactual" conditional with a future time frame, and a "predictive" conditional as described in the previous section, may be slight. For example, there is no great practical difference in meaning between "If it rained tomorrow, I would cancel the match" and "If it rains tomorrow, I will cancel the match".
It is in the counterfactual type of conditional sentence that the grammatical form called the conditional mood (meaning something like the English "would ...") is most often found. For the uses of particular verb forms and grammatical structures in the various types and parts of conditional sentences in certain languages, see the following sections.
Grammar of conditional sentences 
Languages have different rules concerning the grammatical structure of conditional sentences. These may concern the syntactic structure of the condition clause (protasis) and consequence (apodosis), as well as the forms of verbs used in them (particularly their tense and mood). Rules for English and certain other languages are described below; more information can be found in the articles on the grammars of individual languages. (Some languages are also described in the article on the conditional mood.)
In English conditional sentences, the condition clause (protasis) is most commonly introduced by the conjunction if, or sometimes other conjunctions or expressions such as unless, provided (that), providing (that) and as long as. Certain condition clauses can also be formulated using inversion without any conjunction (should you fail...; were he to die...; had they helped us...).
In English language teaching, conditional sentences are often classified under the headings zero conditional, first conditional (or conditional I), second conditional (or conditional II), third conditional (or conditional III) and mixed conditional, according to the grammatical pattern followed.
"Zero conditional" refers to conditional sentences that express a simple implication (see above section), particularly when both clauses are in the present tense: "If you don't eat for a long time, you become hungry." This form of the conditional expresses the idea that a universally known fact is being described: "If you touch a flame, you burn yourself." The act of burning oneself only happens on the condition of the first clause being completed. However such sentences can be formulated with a variety of tenses (and moods), as appropriate to the situation.
"First conditional" refers to predictive conditional sentences (see above section); here, normally, the condition is expressed using the present tense and the consequence using the future: "If you make a mistake, someone will let you know."
"Second conditional" refers to the pattern where the condition clause is in the past tense, and the consequence in conditional mood (using would or, in the first person and rarely, should). This is used for hypothetical, counterfactual situations in a present or future time frame (where the condition expressed is known to be false or is presented as unlikely).
- If I liked parties, I would attend more of them.
- If it rained tomorrow, I would dance in the street.
The past tense used in the condition clause is historically the past subjunctive; however in modern English this is identical to the past indicative except in the case of the verb be (first and third person singular), where the indicative is was and the subjunctive were. In this case either form may be used (was is more colloquial, and were more formal, although the phrase if I were you is common in colloquial language too):
- If I (he, she, it) was/were rich, there would be plenty of money available for this project.
"Third conditional" is the pattern where the condition clause is in the past perfect, and the consequence is expressed using the conditional perfect. This is used to refer to hypothetical, counterfactual (or believed likely to be counterfactual) situations in the past
- If you had called me, I would have come.
"Mixed conditional" usually refers to a mixture of the second and third conditionals (the counterfactual patterns). Here either the condition or the consequence, but not both, has a past time reference:
- If you had done your job properly, we wouldn't be in this mess now.
- If we were soldiers, we wouldn't have done it like that.
A range of variations on the above structures are possible. For details, see the article on English conditional sentences.
Conditional sentences in Latin are traditionally classified into three categories, based on grammatical structure.
- simple conditions (factual or logical implications)
- present tense [if present indicative then indicative]
- past tense [if perfect/imperfect indicative then indicative]
- future conditions
- "future more vivid" [if future indicative then future indicative]
- "future less vivid" [if present subjunctive then present subjunctive]
- contrafactual conditions
- "present contrary-to-fact" [if imperfect subjunctive then imperfect subjunctive]
- "past contrary-to-fact" [if pluperfect subjunctive then pluperfect subjunctive]
In French, the conjunction corresponding to "if" is si. The use of tenses is quite similar to English:
- In implicative conditional sentences, the present tense (or other appropriate tense, mood, etc.) is used in both clauses.
- In predictive conditional sentences, the future tense or imperative generally appears in the main clause, but the condition clause is formed with the present tense (as in English). This contrasts with dependent clauses introduced by certain other conjunctions, such as quand ("when"), where French uses the future (while English has the present).
- In counterfactual conditional sentences, the imperfect is used to express the condition (where English similarly uses the past tense). The main clause contains the conditional mood (e.g. j'arriverais, "I would arrive").
- In counterfactual conditional sentences with a past time frame, the condition is expressed using the pluperfect e.g. (s'il avait attendu, "if he had waited"), and the consequence with the conditional perfect (e.g. je l'aurais vu, "I would have seen him"). Again these verb forms parallel those used in English.
As in English, certain mixtures and variations of these patterns are possible. See also French verbs.
Italian uses the following patterns (the equivalent of "if" is se):
- Present tense (or other as appropriate) in both parts of an implicative conditional.
- Future tense in both parts of a predictive conditional sentence (the future is not replaced with the present in condition clauses as in English or French).
- In a counterfactual conditional, the imperfect subjunctive is used for the condition, and the conditional mood for the main clause. A more informal equivalent is to use the imperfect indicative in both parts.
- In a counterfactual conditional with past time frame, the pluperfect subjunctive is used for the condition, and the past conditional (conditional perfect) for the main clause.
See also Italian verbs.
Slavic languages 
In Slavic languages, such as Russian, clauses in conditional sentences generally appear in their natural tense (future tense for future reference, etc.) However, for counterfactuals, a conditional/subjunctive marker such as the Russian бы by generally appears in both condition and consequent clauses, and this normally accompanies the past tense form of the verb.
While the material conditional operator used in logic (i.e.) is sometimes read aloud in the form of a conditional sentence (i.e. "if p, then q"), the intuitive interpretation of conditional statements in natural language does not always correspond to the definition of this mathematical relation. 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.
See also 
- Anankastic conditional
- English modal auxiliary verb
- Conditional mood
- Counterfactual conditional
- Subjunctive mood
- Haspelmath, Martin; König, Ekkehard; Oesterreicher, Wulf; Raible, Wolfgang: Language Typology and Language Universals, Walter de Gruyter, 2001, p. 1002.
- Mead, Hayden; Stevenson, Jay (1996), The Essentials of Grammar, New York: Berkley Books, p. 55, ISBN 978-0-425-15446-5, OCLC 35301673
- Craig Thane, Teacher Training Essentials: Workshops for Professional Development, Cambridge University Press, 2010, p. 67.