Breakdown of Sinun ei pidä pussittaa valkoista palloa!
sinä
you
ei
no / not (en, et, ei, emme, ette, eivät)
valkoinen
white
pitää
must / have to
Questions & Answers about Sinun ei pidä pussittaa valkoista palloa!
Why is the object valkoista palloa in the partitive case?
Because the sentence is negative. In Finnish, a negative sentence almost always forces the object into the partitive case. Even though pussittaa is an infinitive chained to a necessity verb (pitää), the negation ei at the start of the chain overrides everything else and demands a partitive object.
If the sentence were affirmative ('You must pot the white ball!'), what case would the object take?
It would be in the nominative-looking accusative: Sinun pitää pussittaa valkoinen pallo! When chaining an infinitive to a necessity verb like pitää, the total object normally takes this uninflected form. But as we saw above, the negation in our actual sentence flips it to partitive.
Why does ei pidä mean 'must not' here, rather than 'don't have to'?
While pitää means 'must' or 'have to', its negative form ei pidä translates strictly to 'must not' (a negative obligation). If you want to say 'don't have to' (a lack of obligation), you have to switch verbs and use ei tarvitse.
Why is the subject Sinun in the genitive case?
Necessity verbs like pitää require their subject to be in the genitive case. Even though the main verb is chained with the infinitive pussittaa, it is the necessity verb pitää that dictates the case of the subject.