Step-by-step explanation:
28) Stefan, passt dir das Hemd sa?
We are talking with Stefan so the answer will be in 2nd person. Passen is a dative verb. Ihnen cannot come bcz Stefan isn't someone formal. Dir is the correct answer (dativ form of du).
Tip - To understand if the person is formal, "Herr" or "Frau" will be written before their name or surname (e.g. Herr Schmidt, Frau Meier, etc.)
29) Ich kaufe ihm ein Spiel.
When we ask "what" to this sentence, we get the answer "ein Spiel". The answer comes in Akkusativ.
When we ask "to whom" we get the answer "mein Kind". The answer comes in Dativ.
Kind is 3rd person. Thus, it changes to ihm (dativ form of es)