Quick Guide to Conditionals

There are four conditionals in English. Here is a quick guide. The tenses used in each sentence are in red.

Zero
If you press the button, the light comes on.
* used for inevitable, predictable or natural conditions. Often used in instructions or for describing processes or machines. 'If' could be replaced by 'when'.

First
If you study hard, you will improve.
* used for possible conditions where one action is dependent on another. Also used for promises and intentions. First often overlaps with zero conditional for events or processes which are more certain to happen.
Note: 'Will' can be replaced by other modal verbs such as, in this case, 'should', 'could', 'might' and 'may'.

Second
If I had a million euros, I would start a health-food restaurant.
* used for hypothetical, imaginary or unlikely conditions.
Note: 'Would' can be replaced by other modal verbs, in this case by 'might' or 'could'.

Also note:
For invitations, it is common to use 'would' in the if-clause followed by present or future:
'If you would like more information, please don't hesitate to contact us'.
'If you would like to take a seat, somebody will be with you shortly'.

Third
If I had known about the sheer difficulty of conditionals, I would have chosen another language (!).
* used for regret or impossible conditions.
Note: 'Would have' can be replaced by 'might have' and 'could have'.

There are some useful conditional phrases which do not use if:

Zero conditional
When you press the button, the light comes on.
Pressing the button turns the light on.

First conditional
Provided you study hard, you will improve.
As long as you study hard, you will improve.
Improvementwill depend on whether you study hard or not.

Second conditional
Had I a million euros, I would start a restaurant.
Were I a millionaire, I would start a restaurant.
Supposing I had a million euros. I would start a restaurant.
Assuming I had a million euros, I would start a restaurant.

Third conditional
I wish I had known about the difficulty of conditionals.
If only I had known about the difficulty of conditionals.
Had I known about the difficulty of conditionals, I would not have learned English (!).