I often see signs in Hong Kong buildings that say 'When there is a fire please do not use the lift'. I know this is incorrect, it should be 'if'. Can you explain to me why this is so.
If we use 'when' in this statement, we are suggesting that a fire will certainly happen. If we use 'if' in this statement, we are acknowledging that a fire may possibly happen: much more realistic and comfortable.
We use 'If' statements in conditional structures when we speculate about the future.