adjective
BAD : Be careful to water the African violets regularly.
GOOD : Make sure (that) you water the African violets regularly.
BAD : Be careful to fix any oil leaks.
GOOD : Make sure (that) you fix any oil leaks.
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Use be careful when you want someone to pay special attention to something so that they do not have an accident, make a mistake, or do something that will cause damage: 'Be careful! You're about to spill your coffee.' 'You should be careful about what you say to her. She is easily offended.'
To tell someone that they must not forget to do or check something, use make sure : 'Before you set off, make sure that you have enough petrol.' 'Make sure that you don't leave the key in the car.'