Questions & Answers about Hänen täytyy ostaa uusi biljardikeppi.
Why is uusi biljardikeppi in the nominative case instead of the genitive uuden biljardikepin?
This is a perfect example of object case inheritance. The main verb here is täytyy, which is a necessity verb. Necessity verbs require a total object to be in the nominative case. Even though the object is directly attached to the infinitive ostaa, it inherits the grammatical rule from the "boss" verb of the chain, täytyy.
How would this sentence change if the main verb wasn't a necessity verb, like if we used haluaa (wants to)?
If the main verb is a standard verb, the object case rule defaults back to the normal genitive (often called genitive-accusative) for total singular objects. The sentence would become Hän haluaa ostaa uuden biljardikepin. The subject would also flip back to the nominative Hän.
What if we make this sentence negative? Does the object case inheritance rule still apply?
No, the negative object rule overrides everything else! If we say "He doesn't have to buy...", the main verb becomes negative (ei tarvitse), forcing the object into the partitive case: Hänen ei tarvitse ostaa uutta biljardikeppiä.
Why does the sentence start with the genitive Hänen instead of the nominative Hän?