In case... / Should... / If... (hypothetical)
Introduces a hypothetical condition. Always followed by the subjunctive. More formal than si. En caso de que + subjunctive.
In case you can't come, let me know.
Should there be any problems, call this number.
In case you change your mind, the offer still stands.
In case I'm not here, leave a message.
Take an umbrella in case it rains.