2). Grammar: conditional sentences

As is typical for many languages, full conditional sentences in English consist of a condition clause or protasis specifying a condition or hypothesis, and a consequence clause or apodosis specifying what follows from that condition. The condition clause is a dependent clause, most commonly headed by the conjunction if, while the consequence is contained in the main clause of the sentence. Either clause may appear first.

Different types of conditional sentences (depending largely on whether they refer to a past, present or future time frame) require the use of particular verb forms (tenses and moods) to express the condition and the consequence. In English language teaching the most common patterns are referred to as first conditional, second conditional and third conditional; there is also a zero conditional and mixed conditional. In English conditional sentences, the condition clause (protasis) is a dependent clause, most commonly introduced by the conjunction if. Other conjunctions or equivalent expressions may also be used, such as unless (meaning "if...not"), provided (that), providing (that) and as long as. Certain condition clauses can also be formulated using inversion without any conjunction.

The apodosis, expressing the consequence of the stated condition, is generally the main clause of the sentence. Depending on the sentence type, it may be a statement, question, or order. It may appear before or after the condition clause:

If I see him, I will tell him. (declarative sentence, condition first)

I will tell him if I see him. (declarative sentence, condition second)

If you saw him, would you tell him? (interrogative sentence, condition first)

Would you tell him if you saw him? (interrogative sentence, condition second)

If you see it, photograph it. (imperative sentence, condition first)

Photograph it if you see it. (imperative sentence, condition second)

As with other dependent clauses in English, it is common for a comma to be used to separate the clauses if the dependent clause comes first (as is done in the above examples). It is possible for the consequence clause to appear alone in a sentence, without a condition clause, if the condition has been previously stated or is understood from the context. It may also be shortened by verb phrase ellipsis; a minimal conditional sentence could therefore be something like "Would you?" or "I would."

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, particularly in terms of the verb tenses and auxiliaries used. These patterns and their variations are described in the following sections.

Zero conditional

"Zero conditional" refers to conditional sentences that express a factual implication, rather than describing a hypothetical situation or potential future circumstance. The term is used particularly when both clauses are in the present tense; however such sentences can be formulated with a variety of tenses/moods, as appropriate to the situation: If you don't eat for a long time, you become hungry.

If the alarm went off, there's a fire somewhere in the building.

If you are going to sit an exam tomorrow, go to bed early tonight!

If aspirins will cure it, I'll take a couple tonight.

The first of these sentences is a basic zero conditional with both clauses in the present tense. The last is an example of the use of will in a condition clause^[1]. The use of verb tenses, moods and aspects in the parts of such sentences follows general principles, as described in Uses of English verb forms. Occasionally, mainly in a formal and somewhat archaic style, a subjunctive is used in the condition clause (as in "If the prisoner be held for more than five days, ...). For more details see English subjunctive.