Conditional Clauses

First Conditional: real possibility
We are talking about the future. We are thinking about a particular condition or situation in the future, and the result of this condition. There is a real possibility that this condition will happen. For example, it is morning. You are at home. You plan to play tennis this afternoon. But there are some clouds in the sky. Imagine that it rains. What will you do?
IF
condition
result

present simple
WILL + base verb
If it rains I will stay at home.
Notice that we are thinking about a future condition. It is not raining yet. But the sky is cloudy and you think that it could rain. We use the present simple tense to talk about the possible future condition. We use WILL + base verb to talk about the possible future result. The important thing about the first conditional is that there is a real possibility that the condition will happen. Here are some more examples (do you remember the two basic structures: [IF condition result] and [result IF condition]?):
IF condition result

present simple
WILL + base verb
If I see Mary I will tell her.
If Tara is free tomorrow he will invite her.
If they do not pass their exam
their teacher will be sad.
If it rains tomorrow
will you stay at home?

condition
WILL + base verb

present simple
I will tell Mary
if I see her.
He will invite Tara
if she is free tomorrow.
Their teacher will be sad
if they do not pass their exam.
Will you stay at home
if it rains tomorrow?
What will you do
if it rains tomorrow?


Second Conditional: unreal possibility or dream
The second conditional is like the first conditional. We are still thinking about the future. We are thinking about a particular condition in the future, and the result of this condition. But there is not a real possibility that this condition will happen. For example, you do not have a lottery ticket. Is it possible to win? No! No lottery ticket, no win! But maybe you will buy a lottery ticket in the future. So you can think about winning in the future, like a dream. It's not very real, but it's still possible.
IF condition result

past simple
WOULD + base verb
If I won the lottery I would buy a car.
Notice that we are thinking about a future condition. We use the past simple tense to talk about the future condition. We use WOULD + base verb to talk about the future result. The important thing about the second conditional is that there is an unreal possibility that the condition will happen.
Here are some more examples:
IF condition
result

past simple WOULD + base verb
If I married Mary I would be happy.
If Ram became rich she would marry him.

Sometimes, we use should, could or might instead of would, for example: If I won a million dollars, I could stop working.


Third Conditional: no possibility

The first conditional and second conditionals talk about the future. With the third conditional we talk about the past. We talk about a condition in the past that did not happen. That is why there is no possibility for this condition. The third conditional is also like a dream, but with no possibility of the dream coming true.
Last week you bought a lottery ticket. But you did not win. :-(
condition
result

Past Perfect
WOULD HAVE + Past Participle
If I had won the lottery I would have bought a car.
Notice that we are thinking about an impossible past condition. You did not win the lottery. So the condition was not true, and that particular condition can never be true because it is finished. We use the past perfect tense to talk about the impossible past condition. We use WOULD HAVE + past participle to talk about the impossible past result. The important thing about the third conditional is that both the condition and result are impossible now.
Sometimes, we use should have, could have, might have instead of would have, for example: If you had bought a lottery ticket, you might have won.
Look at some more examples in the tables below:
IF
condition
result

past perfect

WOULD HAVE + past participle
If I had seen Mary I would have told her.
If Tara had been free yesterday I would have invited her.

past perfect
I would have told Mary if I had seen her.
I would have invited Tara if she had been free yesterday.

Zero Conditional: certainty

We use the so-called zero conditional when the result of the condition is always true, like a scientific fact.
Take some ice. Put it in a saucepan. Heat the saucepan. What happens? The ice melts (it becomes water). You would be surprised if it did not.
IF
condition
result

present simple
present simple
If you heat ice it melts.
Notice that we are thinking about a result that is always true for this condition. The result of the condition is an absolute certainty. We are not thinking about the future or the past, or even the present. We are thinking about a simple fact. We use the present simple tense to talk about the condition. We also use the present simple tense to talk about the result. The important thing about the zero conditional is that the condition always has the same result.
We can also use when instead of if, for example: When I get up late I miss my bus.
Look at some more examples in the tables below:
IF
condition
result

present simple
present simple
If I miss the 8 o'clock bus I am late for work.
If I am late for work my boss gets angry.
If people don't eat they get hungry.
If you heat ice does it melt?