conditional sentences

TYPE 1
if + Simple Present, will-Future
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.

USE :
Conditional Sentences 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.
*Example: If I find her address, I’ll 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.
*Example: If John has the money, he will buy a Ferrari.
I know John very well and I know that he earns a lot of money and that he loves Ferraris. So I think it is very likely that sooner or later he will have the money to buy a Ferrari.

TYPE 2
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 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.

USE :
Conditional Sentences Type II 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 I found her address, I would send her an invitation.
Fact: 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.
Fact: 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.

TYPE 3
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.

USE :
Conditional Sentences 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.
Fact: 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.
Fact: 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.

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