1, THE ZERO CONDITIONAL SENTENCE
+) The zero conditional is used when describing situations which have automatic or habitual results. Using this conditional suggests that we are 100% sure of the result.
--- Examples:
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:
---Examples :
Press the button if you want a receipt.
If you want to leave a message, speak after the tone.

2, THE FIRST CONDITIONAL SENTENCE
+) 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.
---Examples :
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.
Note that the meaning here is present or future, but the main verb in the if-clause is in a present, not future tense.
A . Possible variations of the basic form

Sometimes instead of if + present + future, we may have:
a) if + present + may/might (possibility)
If the climate keeps warming, the Arctic might be warm enough for swimming.
b) if + present + may (permission) or can (permission or ability)
If your documents are in order, you may/can leave at once. (permission)
If it stops raining, we can go out." (permission or ability)
c) if + present + must, should or any expression of command, request or advice
if you want to look slim, you must/should eat less meat.
if you want to look slim, you had better eat less meat.
if you want to look slim, eat less meat.

d) When if is used to mean as/since, a variety of tenses can be used in the main clause
B . Variations of the if-clause

Instead of if + present tense, we can have:
a) if + present continuous, to indicate a present actions or a future arrangement."
If you are waiting for a bus (present action), you'd better join the queue.
If you are looking for Peter, you'll find him upstairs.
If you're staying for another night (future arrangement), I'll ask the manager to give you a better room.
b) if + present perfect
if you have finished dinner, I'll ask the waiter for the bill.
If has written the letter, I'll post it.
If they haven't seen the museum, we'd better go there today.

3, THE SECOND CONDITIONAL SENTENCE
+) We use the Second Conditional:
# to give advice
# to talk about a future situation that is unlikely to happen.
***+) The verb in the if-clause is in the past tense; the verb in the main clause is in the conditional tense.
---Examples :
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.)
~~ NOTE :
There is no difference between the first and second conditionals as far as time is concerned. The first conditional, like the second conditional refers to the present or future. The past tense in the if-clause is not a true past but a subjunctive, which indicates improbability or unreality.
~~ USE :
A. When the supposition is contrary to known facts
"If I lived in New York, I wouldn't have to commute there each day." (But I don't live in New York.)
"If I were you, I would plant some trees in your garden." (But I'm not you.)
B. When we don't expect the action in the if-clause to happen:
"If I saw a zombie, I would run as fast as I could." (But I don't expect to see a zombie.)
"If I bought a car like this, everyone would admire me." (But I don't intend to buy the car.)
***+) Possible variations of the basic form
Variations of the main clause might or could may be used instead of would:
If you tried again, you would succeed. (certain result)
If you tried again, you might succeed. (possible result)
If I knew her number, I could ring her up. (ability)
If he had a permit, he could get a job. (ability or permission)
The continuous conditional form may be used instead of the simple:
Peter is on holiday; he is touring Italy. ~ "If I were on holiday I would/might be touring Italy too."

4, THE THIRD CONDITIONAL SENTENCE
+) We use it when talking about a past condition that cannot be fulfilled, because the action in the if-clause didn't happen.
---For example : imagine that you missed a train (and as a result you were late for an important meeting). You could say:
If I hadn't missed the train, I wouldn't have been late for the meeting.
+) Do you know what tenses each of the clauses is in? Let's see:
The verb in the if-clause is in the Past Perfect Tense
The verb in the main clause is in the Perfect Conditional
If-clause (Past Perfect) Main clause (Perfect Conditional)
If I hadn't missed the train, I wouldn't have been late for the meeting.
More examples:
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)
***+) Variations
The form of the conditional can be a little different.
a) could or might may be used instead of would:
If the rescue crew had found him earlier, they could have saved his life. (ability)
If the rescue crew had found him earlier, they might have saved his life. (possibility)
If we had the necessary documents, we could have left at once. (ability or permission)
b) The continuous form of the Perfect Conditional may be used:
If I had had any money I would have been watching the film with my girlfriend that evening.
c) We can use the Past Perfect Continuous in the if-clause:
I wasn't wearing a seat belt. If I had been wearing one, I wouldn't have been seriously injured.
d) A combination of types 2 and 3 is possible:
The airplane I intended to catch crashed. If I had caught that airplane, I would have been killed or I would be dead now(type 3)
If he had worked harder at school, he would be working in a comfortable office now; he wouldn't be sweeping the streets. (But I didn't work hard at school and now he is sweeping the streets.)
e) Using inversion, we can place "had" before the subject, omitting the "if": For example, instead of saying:
If you had obeyed orders this disaster would not have happened.
we can say:
Had you told me about your problems, this disaster would not have happened.

5, MIXED CONDITIONAL SENTENCES
+) Mixed conditionals are those unreal conditional sentences whose time in the if-clause is different than the time in the main-clause. Let's first have a look at unreal conditional sentences:
If she were shorter, she would be more attractive.
I am busy next week. If I had time, I would come to your party.
If they hadn't trained hard, they wouldn't have won.
As you can see, they refer to the same time: the present, the future or the past. If we mix the sentences, we get mixed conditionals.
A---Past and Present
If my father hadn't lost his keys, we wouldn't have to wait until he finds them.
(But my father lost his keys and therefore we have to wait until he finds them)
If I had installed an anti-virus, my computer wouldn't be so slow now.
(But I didn't install an anti-virus and therefore my computer is so slow now)
If our house had been broken into, we would be very sad.
(But our house wasn't broken into and we aren't sad.
B---Past and Future)
If our house had been broken into, we would call the police.
(But our house wasn't broken into and we are not going to call police)
If we had won the lottery last week, we would buy a new sofa today.
(But we didn't win the lottery and we are not going to buy a new sofa today.)
C---Present and Past
If I were smarter, I would have graduated from Stanford.
But I am not smarter and therefore I didn't graduate from Stanford.
If Mary weren't a snob, she wouldn't have had so many parties this year.
But Mary is a snob and therefore she had so many parties this year.
D---Present and Future
If you were more eloquent, you would become a politician.
But I am not more eloquent and I won't become a politician.
If you had more time, I would go to the cinema with you.
But you don't have more time and I won't go to the cinema with you.