IF CLAUSE

Conditional Sentences (sentence presupposition) is a form of complex sentences that we can use when we want to say that something is a result or consequence of the other depending on the situation. 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.

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.

Form: if + Simple Present, will-Future

Example: If I find her address, I’ll send her an invitation.

2.      Conditional Sentence Type 2

→ It is possible but very unlikely, that the condition will be fulfilled.

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 …“

Form: if + Simple Past, Conditional I (= would + Infinitive)

Example: If I found her address, I would send her an invitation.

3.      Conditional Sentence Type 3

→ It is impossible that the condition will be fulfilled because it refers to the past.

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.

Form: if + Past Perfect, Conditional II (= would + have + Past Participle)

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

http://www.ego4u.com/en/cram-up/grammar/conditional-sentences
http://bahasainggrisonlines.blogspot.com/2013/01/conditional-sentences.html