Conditional type zero is used to talk about general truths, scientific facts or things which always happen under certain conditions.
Form:
Use:
The zero conditional is used to talk about things which are always true, scientific facts, general truths:
Examples:
If you cross an international date line, the time changes.
Phosphorus burns if you expose it to air.
If I wake up early, I go jogging.
NOTE: you can use "when" instead of "if".
Use
Conditional Sentences Type 1 refer to the future. An action in the future will only happen if a certain condition is fulfilled by that time. We don't know for sure whether the condition actually will be fulfilled or not, but the conditions seems rather realistic – so we think it is likely to happen.
Form:
Example:
If I have enough time, I'll watch the football match.
I may have time to watch the match but I'm not sure about it.
Often called the "unreal" conditional because it is used for unreal impossible or improbable situations. This conditional provides an imaginary result for a given situation. It is very unlikely that the condition will be fulfilled.
Form:
Were / Was
In conditional type 2, we usually use in the if clause "were" instead of "was" even if the pronoun is I, he, she or it. "were" here is a subjunctive form.
NOTE "was" is also a possible form.
Example:
If I were a millionaire, I would buy a castle.
Use
Conditional Sentences Type 2 refer to an action in the present that could happen if the present situation were different. I don't really expect the situation to change because it is very unlikely.
Example:
If I had a lot of money, I would travel around the world.
It is impossible that the condition will be met because it refers to the past.
Form:
Use
Conditional Sentences Type 3 refer to situations in the past. They express hypothetical results to past given situations.
Example:
If he had been careful, he wouldn't have had that terrible accident.
Sometimes in the past, he was careless. He drove so fast. So he had a terrible accident
1. The main clause can also be at the beginning of the sentence. In this case, don't use a comma.
Examples:
"Phosphorus burns if you expose it to air."
" I will send her an invitation if I find her address."
" I would travel around the world if I had a million dollars."
"He wouldn't have had that terrible accident if he had been careful."
2. Main clause and/or if clause might be negative.
Example:
If I don’t see him this afternoon, I will phone him in the evening.
If he had been careful, he wouldn't have had an accident.