Conditional Sentence

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.

Conditional Sentence Type 1                                                   

→ It is possible and also very likely that the condition will be fulfilled.

Form: if + Simple Present, will-Future

Conditional Sentence Type 2

→ It is possible but very unlikely, that the condition will be fulfilled.

Form: if + Simple Past, Conditional I (= would + Infinitive)

Conditional Sentence Type 3

→ It is impossible that the condition will be fulfilled because it refers to the past.

Form: if + Past Perfect, Conditional II (= would + have + Past Participle)

Example:

Type		Examples
		long forms	short/contracted forms
I	+	If I study, I will pass the exam.	If I study, I'll pass the exam.
	-	If I study, I will not fail the exam. If I do not study, I will fail the exam.	If I study, I won't fail the exam. If I don't study, I'll fail the exam.
II	+	If I studied, I would pass the exam.	If I studied, I'd pass the exam.
	-	If I studied, I would not fail the exam. If I did not study, I would fail the exam.	If I studied, I wouldn't fail the exam. If I didn't study, I'd fail the exam.
III	+	If I had studied, I would have passed the exam.	If I'd studied, I'd have passed the exam.
	-	If I had studied, I would not have failed the exam. If I had not studied, I would have failed the exam.	If I'd studied, I wouldn't have failed the exam. If I hadn't studied, I'd have failed the exam.

 Referensi :

http://www.ego4u.com/en/cram-up/grammar/conditional-sentences

http://en.wikipedia.org/wiki/Conditional_sentence

http://www.englisch-hilfen.de/en/grammar/if.htm