They are in two halves: the first one contains a condition (the "IF'' part) and the other one contains a consequence with words such as WILL, CAN, MAY, MIGHT, COULD or WOULD.
Here is a little chart to help you visualize the different types *:
|100 %||__________________||0 CONDITIONAL||If you heat ice, it melts.||Any time|
|50 %||__________||1st CONDITIONAL||If it rains, I will stay at home.||Future|
|10 %||______||2nd CONDITIONAL||If I won the lottery, I would buy a bigger house.||Future|
|0 %||3rd CONDITIONAL||If I had revised, I would have passed my exams.||Past|
(* Do not take the percentages too literally, they are just here to help you.)
TYPE 1: THE ZERO CONDITIONAL (Certainty)
We use this form when the result is always true, like a scientific fact for instance or general truths.
IF + PRESENT SIMPLE / PRESENT SIMPLE
If it rains, you get wet.
Plants die if they don't have enough water.
We can also use WHEN instead of IF.
TYPE 2: THE FIRST CONDITIONAL (Real Possibility)
In these sentences, there is a strong possibility that the first part (after if) will happen.
IF + PRESENT SIMPLE / WILL + INFINITIVE WITHOUT TO
If his plan succeeds, he will make a huge profit.
If you drop that glass, it will break.
Sometimes, we also use SHALL, CAN or MAY instead of WILL.
TYPE 3: THE SECOND CONDITIONAL (Unreal Possibility / Dream)
In these sentences, the first part with IF shows that the event is unlikely to happen. We often use this type of sentence to talk about hypotheses, or imaginary future events.
IF + PAST SIMPLE / WOULD + INFINITIVE WITHOUT TO
If I was elected President, I would change many things.
If I saw a ghost, I would be scared to death.
* ''Were'' is often used instead of ''was''.
If I were you, I would go to bed immediately.
Sometimes, we use SHOULD, COULD or MIGHT instead of WOULD.
TYPE 4: THE THIRD CONDITIONAL (No Possibility)
The FIRST and the SECOND CONDITIONALS are talking about the future, but with the THIRD CONDITIONAL we are talking about the past. In these sentences, the first part of the sentence with IF didn't happen. So there is no possibility of the second part of the sentence happening. It is alson like a dream, but with no possibility of the dream coming true.
IF + PAST PERFECT / WOULD HAVE + PAST PARTICIPLE
If you had told us you were coming, we would have picked you up from the airport.
If I had won the lottery, I would have travelled around the world.
Sometimes, we use SHOULD HAVE, COULD HAVE or MIGHT HAVE instead of WOULD HAVE.
If you wish to do some exercises, just click HERE.