1- We simply use 'one' in the place of a noun or when we want to avoid repeating a noun (the plural form is 'ones'): - 'I've got three bikes, but I like this ONE best. It's the fastest ONE' 'Yes, that's a good ONE' - I wish you hadn't lost that photo. It was really a good ONE. - Her reaction to the plan is basically a good ONE. - I like the picture. It's really a good ONE. - The students enjoy having her as their teacher. She's really a good ONE. ;) 2- 'Have a good one' and 'Have a nice one' are idioms used to mean 'Have a good morning, afternoon or evening': Thank you. Have a good one. See you tomorrow, John. Have a nice one. 3- It's a slang that means well-done. ;) http://onlineslangdictionary.com/definition+of/good+one It can also be used as an exclamation of delight, approval, etc.; often used ironically: "Oh, good one! You great berk!" (Australian Slang) http://www.slang-dictionary.org/Good_one

If you are talking to those narrow minded teachers, you may be critized for grammatical mistakes. "GOOD ONE" is a slang, it doesn't need to follow the grammatical rules. If you are critized, you need to debate, tell them it is the slang. Good one simply means well done

'good one' is a slang term for well done, congratulations, in response to what some one says to you. Person A: I just passed my math exam. Person B: Oh, good one.

Hello Marc, yes you can say that... you can also use that expression to express if a joke is funny...