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.
1. Conditional Sentence Type 1
It is possible and also very likely that the condition will be fulfilled.
Form : if + Simple Present, will-Future
Example : Example: If I find her address, I will send her an invitation.
The main clause can also be at the beginning of the sentence. In this case, don't use a comma.
Example : I will send her an invitation if I find her address.
Note : Main clause and / or if clause might be negative. See Simple Present und will-Future on how to form negative sentences.
Example : If I don’t see him this afternoon, I will phone him in the evening.
2. 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.
The main clause can also be at the beginning of the sentence. In this case, don't use a comma.
Example : I would send her an invitation if I found her address.
Note : Main clause and / or if clause might be negative. See Simple Past und Conditional I on how to form negative sentences.
Example : If I had a lot of money, I wouldn’t stay here.
Were instead of Was In
IF Clauses Type II, we usually use ‚were‘ even if the pronoun is I, he, she or it .
Example : If I were you, I would not do this.
3. 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.
The main clause can also be at the beginning of the sentence. In this case, don't use a comma.
Example : I would have sent her an invitation if I had found her address.
Note : Main clause and / or if clause might be negative. See Past Perfect and Conditional II on how to form negative sentences.
Example : If I hadn’t studied, I wouldn’t have passed my exams.
bibliography : http://www.ego4u.com/en/cram-up/grammar/conditional-sentences
