Hi Ali, 1. I think you might want to say "What's the advantage of buying a car when you don't have a licence?", right? So you could replace the word "good" with "advantage". 2. You can't say "haven't a car"! You either say "haven't got" (British English) or "don't have" (American English). 3. You can could use if instead of when

"don't have" is more natural than "haven't" in most situations when "have" is the main verb. There are some common and some formal expressions with "haven't" but you are on safe ground if you always choose "don't have". "if" and "when" have the same meaning in this kind of situation. The "if" sentence is a zero conditional. You can check this in grammar resources.

haven't is past tense. example 'I haven't cleaned my room yet'. haven't isn't a verb when you use it. it's a helping verb for the past tense of "to clean". a helping verb isn't the main verb. clean is the main verb. so if you use have as the main verb it would be " don't have" or "do/does not have". hope this helped and good luck!

"What good is..." is a standard expression. "Don't have" is UK English as well.

Thank you