In case

In case expresses the possibility of something happening.
It is used to express the idea of doing something to avoid a possible problem later on.
It gives the reason for an action.

    • Take an umbrella in case it rains.
      = There is a possibility that it will rain, so it’s a good idea to take an umbrella.
    • You’d better take a coat in case it gets cold later on.
      = It could get cold later on, so it might be safer to take a coat.

In case + ‘of’

In case of means ‘if there is’; ‘if (something) should happen’.

    • In case of emergency, call this number.
      = If there is an emergency, call this number.
    • In case of breakdown, please press the red button to call for assistance.
      = If the lift breaks down, please press the red button.

Unless

Unless means ‘except if’ or ‘only if’.
Unless replaces ‘if’ + a negative verb.

    • Sally won’t come unless you invite Harry. Meaning:
      • Sally won’t come except if you invite Harry.
      • Sally will only come if you invite Harry.
      • If you don’t invite Harry, Sally won’t come.
    • We will have a picnic unless it rains. Meaning:
      • We will have a picnic except if it rains.
      • If it doesn’t rain we will have a picnic.
      • If it rains, we won’t have a picnic.