"I buy a watch for Mary."
Translation:Eu îi cumpăr Mariei un ceas.
Ok, now I think the short forms are mandatory with people only, otherwise I could not just "a mănca un măr".
But my question is still, when exactly are the short forms of the pronouns (either accusative or dative) mandatory? I have learnt that "pe Maria" in my second sentence is not enough but needs the "o". So why is "Mariei" fine without preceding "îi" here?