Use "geen" to negate a noun with no article or with "een" ("Ik heb geen tijd"); use "niet" for everything else — verbs, adjectives, definite nouns, adverbial phrases.
Klant: ?
Bakker: , . .
Collega: ?
Sara: , .
Klant: Heeft u nog brood van vanmorgen?
Do you still have bread from this morning?
Vul het ontbrekende woord in:
Sorry, we have no more bread. Tomorrow from seven o'clock again.
“Nee, ik heb er geen.”
No, I don't have one.
“Er zit geen selderij in de soep.”
There is no celery in the soup.
“Ik voel me niet lekker.”
I don't feel well. I have a fever and a headache.
“Niet zo goed.”
Not so well.
“Geen probleem.”
No problem.