If Conditional

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.

Conditional Sentence Type 1 :
It is possible and also very likely that the condition will be fulfilled.
Form : if + Simple Present, will-Future
Example : If I find her address, I'll 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.
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 

Conditional Sentence Type 2 :
It is possible but very unlikely, that the conditional will be fulfilled.
From : If + Simple Past, Conditional I (=would + Infinitive)
Example : If I found her address, I would send her 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.
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 …“

Conditional Sentence Type 3 :
It is impossible that the conditional 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.
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.

Source : https://www.ego4u.com/en/cram-up/grammar/conditional-sentences