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
