"I buy a watch for Mary."

Translation: Eu îi cumpăr Mariei un ceas.

[Schattenparker]

Eu cumpăr Mariei un ceas was accepted. So the unstressed dative pronoun îi is not necessary here.

But do I recall correctly that with accusative the unstressed pronoun o is mandatory, like in:

O văd pe Maria ?

[JamesTWils]

If that's the case, should there be an accusative pronoun for un ceas?

[Schattenparker]

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?

[Mapka15]

"Eu cumpăr un ceas Mariei" was accepted (without îi)...???

[Mapka15]

"Eu cumpăr un ceas pentru Maria." is wrong?