We use when for things that are certain to happen in the future:

• I'll buy you a pair of gloves when I go shopping this afternoon.
  I'll give you a ring when I get home from school.

Note that although the reference is clearly to the future, we use the present tense in the when-clause.

We use if for things that may happen and which we are not certain about:

• If I decide to come to London this year, I'll come and visit you.
  
  
• I'm going to call the police if she's not back within the hour.
  
  
• If you make the salads, I'll prepare the barbecue.

In all of these examples, we are talking about future conditions and whether we use when or if depends upon the certainty of things happening.

However, when and if are interchangeable when we are talking about general conditions that always apply when if means almost the same as whenever. Compare the following:

• I keep the air-conditioning on at night if the temperature goes above 30 degrees.
  
  
• I keep the air-conditioning on at night whenever / when the temperature goes above 30 degrees.
  
  
• If the green flag is flying, it's quite safe to swim here
  You can swim here whenever / when / provided the green flag is flying.
  

In case

We use in case to talk about taking precautions, doing something because something else might happen:

• During the drought, keep the bath filled with water - just in case it is needed.
  
  
• I've bought some candles in case there are more electricity cuts.
  
  
• In case I forget, remind me to check the tyre pressure before we load up the car.

In case or if?

When we use in case we are mostly describing future possible situations. When we use if we are talking about conditions that apply. Compare the following and note the differences in meaning.

• I'll fill up the car with petrol in case you need to go to Brighton. ( = I'll fill up now, because you might need it later.)
  
  
• I'll fill up the car with petrol if you need to go to Brighton. ( = Let me know if you need to go to Brighton and then I'll fill up with petrol.)
  
  
• Take the mobile phone with you in case the car breaks down.
  If the car breaks down, give me a ring.

in case of

Note that in case of is a prepositional phrase used with a noun which is similar in meaning to an if-clause. Compare the following:

• In case of fire, exit from the building by the stairs. Do not attempt to use the lift.
  
  
• If there is a fire, leave the building via the stairs. Don't try to use the lift.
  
  
• In cases of difficulty, phone this help line.
  If you experience any problems, phone this help line.