Conditional sentence
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 consequence or apodosis.
If it rains [condition], (then) the picnic will be cancelled [consequence].
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.
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.
Example : If I were the king of the world, I would make it easy to write English.
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.
Example : If I'm your teacher next year, I'll give you some writing exercises.
