Conditional clauses are also called conditional sentences or if sentences. They always have two parts, a main clause and an if clause. These two parts are closely connected. The action in the main part can only happen if a certain condition is taking place. This condition has to be expressed in the if clause.
If he comes to my party, I will be happy.
The condition is "If he comes to my party" and the consequence of this condition is "I will be happy".
You can start a conditional clause with the main clause or the if clause:
If I have a problem, Susan always helps me. or Susan always helps me if I have a problem.
The zero conditional is used with general statements, actions that are certainly happening and facts. We use it with things that are true. In both clauses the present tense is used.
If there is a problem, I can always talk to Carol.
Present Tense ------ Present Tense
First Conditional - Type I
It's possible that . . . We use the Conditional I to talk about the future situations that are realistic to happen. We use the present tense in the if-clause and the will-future in the main clause.
Colin is an intelligent boy. It is April 15th. The exam is on April 23rd. He has enough time to learn for the exam - he can pass it.
If he studies, he will pass the exam on April 23rd.
Present Tense ------ Will Future
Second Conditional - Type II
It's possible but not very probable . . . We use the Conditional II to talk about the future situations that are unreal or nearly impossible to happen. We use the past tense in the if-clause and conditional present in the main clause.
Colin didn't learn. He played football. It' April 22nd. Tomorrow is the exam. It is possible but not very probable that he will pass the exam.
If he studied, he would pass the exam.
Past Tense ------ Conditional Present (would + infinitive)
Third Conditional - Type III
It didn't happen and it is impossible now . . . We use the Conditional III to talk about past situations that didn't happen. We use the past perfect tense in the if-clause and the conditional perfect in the main clause.
It's April 23rd. Colin didn't pass the exam.
If he had learned, he would have passed the exam.
Past Perfect ------ Conditional Perfect (would + have + 3rd form)
Using commas in conditional sentences
When the condition is at the beginning of the sentence, you have to separate it from the main clause with a comma. If the condition is at the back of the sentence, you don't use a comma.
Examples: If he comes to my party, I'll be happy. I'll be happy if he comes to my party.