Conditional Sentences / If-Clauses Type I, II, and III

    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