If Clauses (Conditional Sentences)

Conditional Sentences are also known as Conditional Clauses or If Clauses. They are used to express that the action in the main clause (without if) can only take place if a certain condition (in the clause with if) is fulfilled. There are three types of Conditional Sentences

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. Sometimes this type known as unlikely & impossible conditions
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://www.ego4u.com/en/cram-up/grammar/conditional-sentences