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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!
be careful not to (do something)
Proceed cautiously, in an attempt to avoid potential problems or pitfalls. This phrase is often said as a warning or instruction. I was careful not to make any noise as I crept into the house after curfew. As you plan your talk, be careful not to bore the audience with scientific terms they won't understand. This plate is very hot—be careful not to burn yourself.
you can't be too careful
One must exercise the utmost caution and prudence (in the face of a very dangerous, risky, or problematic thing or situation). I'm sure everything is in order, but I'd like to have my lawyer read over the contract one more time—you can't be too careful. I think you should wear one of those holsters that keeps your passport and some extra cash hidden under your clothes. You can't be too careful when you're traveling abroad alone, you know.
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.
careful not to do something
using care to avoid some difficulty or difficulties. Please be careful not to discuss politics with Mr. Brown. Please be careful not to leave the house unlocked.
careful (with something)
treating or handling something with caution. Please be careful with the vase. It's old and valuable. That's too big a load. Be careful!
If you can't be good, be careful.
Prov. If you are going to do immoral things, make sure they are not dangerous.; If you are going to do something immoral, make sure to keep it secret. (Sometimes used as a flippant way of saying good-bye.) Be a good girl on your vacation trip. Or if you can't be good, be careful. Ernest likes to close his letters with, "If you can't be good, be careful."