Conditional Clauses

0. Talking about general truth:
if clause = present
main clause = present

Example :
If you break the law you are an offender.
There is not really a condition linked to this statement. It is true for everyone.

If they lose weight during the medical therapy, they regain it afterwards.
General statement based on experience.

1. Expressing possibility:
if clause = present
main clause = will future

Example :
If the weather is as sunny tomorrow as it was today, we will go surfing.
Talking about things which may possibly happen in the future.

I will call you if I need any help.
Referring to a possible future event.

2. Expressing unlikability:
if clause = past
main clause = conditional I

Example :
If I had any money I would lend it to you.
Talking about things that are unlikely to happen.

They wouldn't believe me if I told them the truth.
Expressing personal doubts.

If I were you I wouldn't do this.
Mind : "were" is often used instead of "was" esp. after "I".

3. Expressing impossibility:
if clause = past perfect
main clause = conditional II

Example :
If I had seen the car earlier, I wouldn't have stepped onto the road.
Talking about things in the past that could have happened, but that didn't actually happen.

If I had seen the car earlier, I wouldn't have stepped onto the road.
Talking about things in the past that could have happened, but that didn't actually happen.

I would have called him if I had known he was so depressed.
Impossible to change because over.




Source : Grammar Tutorial