Conditional Sentences (IF-Clauses)

Conditional Sentences / If-Clauses 

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 :

IF Clause Type I

=> refer to the future. An action in the future will only happen if a certain condition is fulfilled by that time. We don't know for sure whether the condition actually will be fulfilled or not, but the conditions seems rather realistic- so we think it is likely to happen.

Form :            if + Simple Present, will-Future

Example : If I find her address, I will send her an invitation.
=> I want to send an invitation to a friend. I just have to find her address. I am quite sure, however, that I will find it.

=> The main clause can also be at the beginning of the sentences. In this case, don't use a comma.
=> I will send her an invitation if I find her address

Note : Main clause and / or if clause might be negative. See Simple Presend and will-Future on how to form negative sentences.
Example  : If I don't see him this afternoon, I will phone him in the evening.

IF-Clause Type II

ð      => refer to situations in the present. An action could happen if the present situation were different. I don’t really expect the situation to change, however. I just imagine,” what would happen if ..”

example : If Ifound her address, I would send her an invitation.

I would like to send an invitation to a friend. I have looked everywhere for her address, but I cannot find it. So now I think it is rather unlikely that I will eventually find her address.

Example: If John had the money, he would buy a Ferrari.

I know John very well and I know that he doesn't have much money, but he loves Ferraris. He would like to own a Ferrari (in his dreams). But I think it is very unlikely that he will have the money to buy one in the near future.

Form

if + Simple Past, main clause with 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 and 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.

IF-Clause Type III
=> refer to situations in the past. An action could have happened in the past if a certain condition had been fulfilled. Things were different then, however. We just imagine, what would have happened if the situation had been fulfilled.

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

Sometime in the past, I wanted to send an invitation to a friend. I didn't find her address, however. So in the end I didn't send her an invitation.

Example: If John had had the money, he would have bought a Ferrari. 

I knew John very well and I know that he never had much money, but he loved Ferraris. He would have loved to own a Ferrari, but he never had the money to buy one.

Form

if + Past Perfect, main clause with Conditional II

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.

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