If, provided are both used to introduce conditional clauses.
When merely a possibility which may or may not be true is expressed, if is the usual conjunction.
- I’ll pay you double if you get the work finished by Friday.
- We’ll have the party in the garden if the weather’s good. If not (= if the weather is not good), it’ll have to be inside.
- If anyone calls, just say I’ll be back in the office at four o’clock.
- If she hadn’t called, I wouldn’t have known.
- I wouldn’t work for them (even) if they paid me twice my current salary.
- We’ll deal with that problem if and when it arises.
- If disturbed, the bird may abandon the nest, leaving the chicks to die.
When the clause which follows names a stipulation or proviso, provided (or sometimes providing ) is the usual form.
- We’ll buy everything you produce, provided of course the price is right.
- Provided that you have the money in your account, you can withdraw up to £100 a day.