"in case" and "if"

Precautions

In British English, in case is mostly used to talk about precautions – things which we do in order to be ready for possible future situations.

• I always take an umbrella in case it rains. (= … because it might rain.)

To talk about the future, we use a present tense after in case (see here).

• I’ve bought a chicken in case your mother stays to lunch. (not … in case your mother will stay …)

in case … happen to

We can use the expression happen to after in case to emphasise the meaning of ‘by chance’.

• I’ve bought a chicken in case your mother happens to stay to lunch.
• We took our swimming things in case we happened to find a pool.

Should can be used in the same way in British English, but this is no longer very common.

• … in case your mother should stay to lunch.

in case and if

In British English, 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 George comes. (= Let’s buy some wine now because George might come later.)
  Let’s buy a bottle of wine if George comes. (= We’ll wait and see. If George comes, then we’ll buy the wine. If he doesn’t we won’t.)
• I’m taking an 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.)

In American English, in case is often used in a similar way to if.

• In case you’re free this evening, give me a call. (less typical of British English)

in case of

The prepositional phrase in case of has a wider meaning than the conjunction in case, and can be used in similar situations to if in both American English and British English.

• In case of fire, break glass. (= If there is a fire …)