Conditional sentences

 The ZERO CONDITIONAL is used when describing situations which have automatic or habitual results (=general truths). The probability of the result is 100%.
• If you heat ice, it melts." (will melt is also possible)
• If there is a shortage of any product, prices of that product go up.
• My parents get angry if I come home late.
As you can see, both the main clause and the if clause are in the Present Simple.
The Zero Conditional is often used to give instructions:
• Press the button if you want a receipt.
• If you want to leave a message, speak after the tone.
 We use the FIRST CONDITIONAL to talk about a future situation that is possible.
The verb in the if-clause is in the present tense; the verb in the main clause is in the future simple. It doesn't matter which comes first. There is usually a comma between the two clauses.
• If you try very hard, you'll see the difference.
• John will be late, if you don't lend him your car.
This type of sentence implies that the action is very probable.
We use the First Conditional to talk about a future situation that is possible.
The verb in the if-clause is in the present tense; the verb in the main clause is in the future simple. It doesn't matter which comes first. There is usually a comma between the two clauses.
• If you try very hard, you'll see the difference.
• John will be late, if you don't lend him your car.
This type of sentence implies that the action is very probable.
 We use the SECOND CONDITIONAL to talk about a future situation that is unlikely to happen and to give advice.
The verb in the if-clause is in the past tense; the verb in the main clause is in the conditional tense:
• If someone stole my bag, I would immediately contact the police. (But I don't think that anyone will try to steal the bag. The meaning here is future.)
We use the second conditional to talk about a future situation that is unlikely to happen and to give advice.
The verb in the if-clause is in the past tense; the verb in the main clause is in the conditional tense:
• If someone stole my bag, I would immediately contact the police. (But I don't think that anyone will try to steal the bag. The meaning here is future.)
• If I had money, I would bid on this auction. (But I don't have the money. The meaning here is present)
 And here comes the THIRD CONDITIONAL. What's important to remember about this conditional?

a) The verb in the if-clause is in the past perfect tense

b) The verb in the main clause is in the perfect conditional.

c) The time is past and the condition cannot be fulfilled because the action in the if-clause didn't happen.
• If I had known that you were coming, I would have met you at the railway station.
But I didn't know that you were coming so I didn't come.
• If he had tried to leave the country, he would have been stopped at the frontier.
But he didn't try.