be careful
To act cautiously. Usually issued as a warning. Be careful when you go out in the city at night. Be careful with that box—don't bang around my glassware! Be careful! Stay with the tour guide!
if you can't be good, be careful
If you can't stop yourself from behaving improperly, try to limit the degree of risk or danger involved. If you can't be good, be careful, OK? Please try not to get into too much trouble.
Be careful
1. an instruction to take care in a particular situation. Bill: I'm going to the beach tomorrow. Sally: Be careful. Use lots of sunscreen! Jane: Well, we're off to the Amazon. Mary: Heavens! Be careful!
2. a way of saying good-bye while cautioning someone to take care. John: See you around, Fred. Fred: Be careful. Alice: Well, I'm off. John: Bye, Alice, be careful.