TYPE I: Real in the present or future – Actions or events in the future which are likely to happen or have a real possibility of happening.
If + Simple Present, will/can/shall/may + Bare Infinitive
ex: If it rains tomorrow, I will stay at home.
(I think there is a real possibility of rain tomorrow. In this condition, I will stay at home)
If you come early, you can see my mother before she leaves
(It is possible for you to see my mother)
TYPE II: Unreal in the present – Situations or actions in the present or future, which are not likely to happen or are imaginary, hypothetical or impossible.
If + Past Simple/Past Continuous, could/would/should/might + Bare Infinitive
ex: If I won the lottery, I would travel around the world and buy a castle in Norway.
(I think that it is very unlikely that I will win the lottery.)
If I wasn’t watching TV now, I would be playing football.
(I am watching TV, and imagining what I would do if I wasn’t)
TYPE III: Unreal in the past
If + Past Perfect, would have + Past Participle
ex: If I had done my homework, my teacher wouldn’t have shouted at me.
(In reality, I didn’t do my homework, and my teacher shouted at me.)
She would have passed her exam if she had studied more.
(In reality she didn’t study enough, and so she didn’t pass her exam.)
TYPE O: Generally true – Facts or situations which are always true
If + Simple Present, Simple Present
ex: If you heat water, it boils.
If people don’t eat or drink, eventually they die.
MIXED I: Present result of a past condition – Refer to an unreal past condition and its probable result in the present.
If + Past Perfect, would + Bare Infinitive
ex: If it hadn’t rained, I would be at the beach.
(In reality, it rained in the past, and I am not at the beach now)
If I had accepted that job I would be a millionaire now.
(In reality, I didn’t accept that job in the past, and I am not a millionaire now.)
MIXED II: Past result of a present or continuing condition – Refer to an unreal present situation and its probable (but unreal) past result.
If + Simple Past Perfect, would have + Past Participle
ex: If I didn’t love him I wouldn’t have married him.
(I love him and that is why I married him.)
If I were a good cook, I would have invited them to lunch.
(I am not a good cook so I can’t invite them to lunch.)