You use the conjunction "if" when you think of condition.

You use the conjunction "when" when you think of time.

Of course, there is a gray zone where both conjunction are possible.

• Taste the soup and add salt and pepper if necessary (condition).

• Watch the pot and tell me when the water boils (time).

You use the conjunction "if" when you think of condition.

You use the conjunction "when" when you think of time.

Of course, there is a gray zone where both conjunctions are possible.

• Taste the soup and add salt and pepper if necessary (condition).

• Watch the pot and tell me when the water boils (time).