In case and if
In case and if are normally used in quite different ways.
“Do A in case B happens” means “Do A (first) because B might happen later”
“Do A if B happens” means “Do A if B has already happened”. Compare:
Let’s buy a bottle of wine in case Roger comes.
(= Let’s buy some wine now because Roger might come later.)
Let’s buy a bottle of wine if Roger comes.
(=We’ll wait and see. If Roger comes, then we’ll buy the wine. If he doesn’t we won’t.)
–I’m taking the umbrella in case it rains.
I’ll open the umbrella if it rains. (NOT I’ll open the umbrella in case it rains.)
–People insure their houses in case they catch fire. (NOT …if they catch fire.)
People telephone the fire brigade if their houses catch fire. (NOT … telephone… in case their houses catch fire.)
(From Michael Swan, Practical English Usage, Oxford)
