Conditional sentence

Conditional sentences are sentences that express one thing contingent on something else, e.g. "If it rains, the picnic will be cancelled". They are so called because the impact of the main clause of the sentence is "conditional" on the dependent clause. A full conditional thus contains two clauses: the dependent clause expressing the condition, called the "antecedent" (or "protasis"); and the main clause expressing the consequence, called the "consequent" (or "apodosis").
Languages use a variety of grammatical forms and constructions in conditional sentences. The forms of verbs used in the antecedent and consequent are often subject to particular rules as regards their tense, aspect, 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. Many of these categories are visible cross-linguistically.
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"):
They can also be used for logical deductions about particular circumstances (which can be in various mixtures of past, present and future):
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).
Indicative and counterfactual.
One of the most discussed distinctions among conditionals is that between "indicative" and "counterfactual" conditionals:
These examples differ in both form and meaning. The indicative example uses the present tense form "is" in both its antecedent and consequent, while the counterfactual example uses the past tense form "was" in the antecedent and the modal "would" in the consequent. The counterfactual example conveys that the speaker thinks it isn't raining in New York, while the indicative example suggests that the speaker is agnostic about that possibility.
Linguists and philosophers of language sometimes avoid the term "counterfactuals" because not all examples express counterfactual meanings. For instance, the "Anderson Case" has the characteristic grammatical form of a counterfactual conditional, but does not convey that its antecedent is false or unlikely.
The term "subjunctive" has been used as a replacement though it is also acknowledged as a misnomer. Many languages do not have a subjunctive (e.g. Danish and Dutch) and many that do have it don’t use it for this sort of conditional (e.g. French, Swahili, all Indo-Aryan languages that have a subjunctive). Moreover, languages that do use the subjunctive for such conditionals only do so if they have a specific past subjunctive form. 
Recently the term "X-Marked" has been used as a replacement, with indicative conditionals renamed as "O-Marked" conditionals.
Speech act conditionals.
"Biscuit conditionals" (also known as "relevance" or "speech act" conditionals) are conditionals where the truth of the consequent does not depend on the truth of the antecedent.
In "Metalinguistic conditionals", the antecedent qualifies the usage of some term. For instance, in the following example, the speaker has unconditionally asserted that they saw the relevant person, whether or not that person should really be called their ex-husband.
Non-declarative conditionals.
In "conditional questions", the antecedent qualifies a question asked in the consequent.
In "conditional imperatives", the antecedent qualifies a command given in the consequent.
Crosslinguistic variation.
Languages have different rules concerning the grammatical structure of conditional sentences. These may concern the syntactic structure of the antecedent and consequent clauses, 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.)
Latin.
Conditional sentences in Latin are traditionally classified into three categories, based on grammatical structure.
French.
In French, the conjunction corresponding to "if" is "si". The use of tenses is quite similar to English:
As in English, certain mixtures and variations of these patterns are possible. See also French verbs.
Italian.
Italian uses the following patterns (the equivalent of "if" is "se"):
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.
See Russian grammar, Bulgarian grammar, etc. for more detail.
Logic.
While the material conditional operator used in classical logic is sometimes read aloud in the form of a conditional sentence, the intuitive interpretation of conditional statements in natural language does not always correspond to it. Thus, philosophical logicians and formal semanticists have developed a wide variety of conditional logics which better match actual conditional sentences and actual conditional reasoning.