In 'zero' conditional sentences, the tense in both parts of the sentence is the simple present:

In these sentences, the situation is real and possible. They are used to speak of things that usually happen or to show a logical consequence.They often refer to general truths, such as scientific facts.

First Conditional

In first conditional sentences, the tense in the 'if' clause is the simple present, and the tense in the main clause is the simple future

In these sentences, the time is the present or future and the situation is real. They refer to a possible condition and its probable result. They are based on facts, and they are used to make statements about the real world, and about particular situations. When we use this type of conditional we mean that what we say will probably happen

There are other possibilities of verb tense combination:

Condition: present - Main clause: may, can or imperative instead of WILL: "Don't go out this evening if you have an exam tomorrow" / "He may stay overnight if he doesn't have to work" tomorrow

Second Conditional

In second conditional sentences, the tense in the 'if' clause is the simple past, and in the main clause we use: would / could / might + infinitive

In these sentences, the time is present or any time, but the situation is unreal. They are not based on facts, and they refer to an unlikely or hypothetical condition and its probable result. We use this conditional to speak about very unlikely or improbable situations

Third Conditional

The tense in the 'if' clause is the past perfect, and the tense in the main clause is the perfect conditional (would /could / might + have + past participle)

In these sentences, the time is past, and the situation is contrary to reality. The facts they are based on are the opposite of what is expressed. Things cannot be changed because they happened in the past. This can also be used to express regret about something or to reproach somebody for something.