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 such 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]. Syntactically, the condition is the subordinate clause, and the consequence is the main clause. However, the properties of the entire sentence are primarily determined by the properties of the protasis (condition) (its tense and degree of factualness).

English conditional sentences can be divided into the two broad classes of factual/predictive and hypothetical (counterfactual), depending on the form of the verb in the condition (protasis). The terms "factual" and "counterfactual" broadly correspond to the linguistic modalities called realis and irrealis. [edit]Factual 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, often called "first conditional" or "conditional 1". 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 condition. The potential or indicative conditional, often referred to as the "first conditional" or "conditional 1", 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 a past tense: If she had taken that flight yesterday, she would have arrived at 10pm. If she had taken that flight yesterday, she would be with us now. If she took that flight yesterday, she is somewhere in town today. If she took that flight yesterday, we will 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 be picked 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 usually used in the condition clause (protasis) in English: *If it will rain this afternoon, … *If it may have rained yesterday, … There are exceptions, however, in which will is used exactly as in the first example, namely when the action in the if clause takes place after that in the main clause: (The weather forecast says it's going to rain.) Well, if it will rain, we must take umbrellas. If aspirins will cure it, I'll [I will] take a couple tonight instead of this horrible medicine.[1] Other situations in which will can be used in an if clause include when will is not being used as an auxiliary verb, in other words when it is being used modally to express willingness, persistence, or a wish: If you'll [you will] just hold the door open for me a moment, I can take this table out to the kitchen. If you will keep all the windows shut, of course you'll get headaches. If you will excuse me, I think I will slip into something more comfortable.[1][2] 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".