Conditional sentence

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).

Conditional Sentence Type 1

→ It is possible and also very likely that the condition will be fulfilled.
Form: if + Simple Present, will-Future

Example: If I find her address, I’ll send her an invitation.

Conditional Sentence Type 2

→ It is possible but very unlikely, that the condition will be fulfilled.
Form: if + Simple Past, Conditional I (= would + Infinitive)

Example: If I found her address, I would send her an invitation.

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: If I had found her address, I would have sent her an invitation.

Source:
http://en.wikipedia.org/wiki/Conditional_sentences
http://www.ego4u.com/en/cram-up/grammar/conditional-sentences