There are four basic types of conditional sentences in the English language.
Zero conditional (type 0):
I take my umbrella if it rains.
First conditional (type 1):
I'll call you if I work late.
Second conditional (type 2):
If the bus didn't arrive on time, I would drive you to the airport.
Third conditional (type 3):
She wouldn't have come if I hadn't invited her.
Zero conditional sentences (type 0)
The zero conditional describes situations that are always true. If has the same meaning as when or whenever.
If I go to school, I get up at seven. (Whenever I go to school I get up at the same time.)
If you park your car on double yellow lines, you pay a fine. (Whenever you park illegally, you pay a fine.)
We use the present simple tense in both the main clauses and the if clauses.
First conditional sentences (type 1)
The first conditional sentences are used to speculate about possible situations that can really happen at present or in future. We do not use will in the if clause to describe future activities (compare it to time clauses).
If he studies hard, he'll pass the exams.
If we catch the 10.15 train, we will arrive on time.
If you don't get the ticket, what will you do?
We make if clauses with if + present tense and main clauses with will + bare infinitive.
Second conditional sentences (type 2)
In the second conditional sentences we speculate about situations that will probably never happen at present or in future.
If I had more time, I would help you. (But I am not free at the moment. I can't help you).
If I won a million dollars, I would start a business of my own. (But I know that it is not realistic.)
We make if clauses with if + past tense and main clauses with would + bare infinitive.
Note: the verb to be can be specific in the if clause.
If I were rich, I wouldn't work. If he were younger, he would marry her.
(But was is also possible: If I was rich, I wouldn't work. If he was younger, he would marry her.)
But: If I were you, I wouldn't do it. (In this expression, were is much more usual than was.)
The main difference between the first conditional and the second conditional is about probability: the first conditional is realistic, the second conditional is unrealistic.
Sometimes we can use both with the following difference in meaning.
If I see him, I will tell him. (I suppose I will see him, because we go to the same school.)
If I saw him, I would tell him. (I don't think I will see him, because he is ill.)
If I need your help, I'll call you. (It is probable that I will need your help.)
If I needed your help, I'd call you. (It is not very probable that I will need your help.)
Sometimes we must use either the first conditional or the second conditional to make it clear that the situation is real or unreal.
If you get up late, you will miss your bus. (A real situation.)
If I came from your country, I would understand your problems. (An unreal situation - I am not from your country.)
Third conditional sentences (type 3)
The third conditional sentences always refer to the past. We speculate about situations that happened or did not happen in the past.
If I had won a million, I would have started a business of my own. (But I didn't win anything.)
If he had met her, he would have told her. (Unfortunately, he didn't meet her.)
If we hadn't practised, we wouldn't have won the match. (But we practised and won.)
We make if clauses with if + past perfect and main clauses with would + perfect infinitive (have + past participle).
Other forms
Apart from the basic structures described above, we can also make different combinations.
Examples - type 1:
If you have finished your dinner, you can ask for the bill.
If you are feeling tired, take a rest.
If he is a good skier, he might make it.
If you want to be slim, you should eat less.
If you meet her, could you let me know?
Examples - type 2:
If I knew his address, I might go and see him.
If we were on holiday, we would be lying on a beach now.
Why are we watching this film? If we were watching the news, it would be more interesting.
Examples - type 3:
We didn't save any money. If we had saved some money, we might have bought the house.
She wasn't there and I wasn't sitting next to her. But if she had been there, I would have been sitting next to her.
I was looking at the trees when I fell off the bike. If I hadn't been looking at the trees, I wouldn' t have fallen off the bike.
Mixed conditionals
In the mixed conditional sentences we can combine the second and third conditional.
If he had left immediately, he would be here now. (He didn't leave immediately and isn't here.)
If I had studied hard when I was young, I wouldn't be a porter now. (I didn't study and I am a porter.)
If we hadn't told him the way while he was preparing for his journey, he would get lost now. (We told him and he isn't lost.)
Inverted conditionals
We can also make conditional sentences by changing the word order in the if clause.
Had he booked the hotel room, he wouln't have slept at the camp. (If he had booked... )
Were I in your position, I would accept it. (If I were ... )
This is less common, quite formal and is mostly used in writing.
Note
If is the most frequent expression in the if clauses, but other expressions are also possible: even if, provided (that), unless, on condition (that), as long as.
Examples:
You will leave tonight even if you don't want to.
You can have your birthday party provided that you aren't noisy.
We'll sell you the ranch on condition you pay in cash.
Unless you do something, she won't come back. (If you don't do anything, ... )