Conjunctions

If, in case

If introduces the condition for something happening.
In case introduces the reason why somebody takes precautions.

• If the baby wakes up, feed her.
• In case the baby wakes up, I've left some milk in her bottle.

Note that in the sentence If the baby wakes up, feed her, the feeding takes place after the baby wakes up.

In the sentence In case the baby wakes up, I've left some milk in her bottle, leaving the milk in the bottle happens before the baby wakes up.