Conditional structures can be very complicated and complex. But in their simplest forms follow these basic rules.
condition clause / result clause
You can reverse the position of the clauses and they mean the same.
result clause / condition clause
The possibility of each conditional changes from always true to no possibility.
Zero – an eternal truth
pres. simple + pres. simple
If you drink poison you die.
First – an imagined present with a future result – possible
pres. simple + will/modal
If I study hard I will/might/could/should pass my exam.
Second – an imagined present with a present result – low possibility
past simple + modal
If I studied harder I would/might/could/should be smarter.
Third – an imagined past with a past result – impossible
past perfect + would/could/might + present perfect
If I had studied harder I would/might/could have been smarter.
We have alternatives to if… as long as, even if, whether, providing, unless.
The following sentences all mean the same and have the same result – no rain = beach
If it doesn’t rain I will go to the beach.
As long as it doesn’t rain I will go to the beach.
Providing it doesn’t rain I will go to the beach.
I will go to the beach unless it rains.
But these do not – no rain/rain = beach
I will go to the beach whether it rains or not.
Even if it rains I will go to the beach.
We very often use if/whether in non conditional structures but we understand that behind the structure there is an unspoken condition that we are not being told.
That job in China is great but whether (or not) I go is another thing.
That job in China is great but If I go is another thing.