"Do you need a shirt?"
Why is the word "koszula" in genitive (koszuli) in this sentence? I would write "koszulę", since it's singular (a shirt), ends on -a in nominative singular, and is the direct object of the sentence (which I have learned should be accusative). The only explanation I can think of is that the verb is the kind that can only be connected with genitive - could this be the case?. It's really bugging me, since I keep making this mistake.
You are right, the verb "potrzebować" is just used with the genitive and it often depends just on the verb:
- "Potrzebuję koszuli" = "I need a shirt" (or "Nie mam koszuli" = 'I don't have a shirt") (genetiv)
- "Przyglądam się koszuli" = "I look at a shirt" (dative)
- "Widzę koszulę" = "I see a shirt" (accusative)
- "Mówię o koszuli" = "I speak about a shirt" (locativ)
"potrzebujecie" (for plural 'you') was another starred answer, so this must have been what the algorithm wanted from you. But as it's about needing one shirt, it's less probable, so I made it just an acceptable option, not a starred one. "potrzebujesz" should be there next time.