Standard Verbs vs. Necessity Verbs
When a sentence contains a chain of verbs, the first verb in the chain (the main verb) determines the grammatical case of the object.
With standard verbs like aikoa (to intend) or voida (to be able to), a complete object takes the genitive case, ending in -n.
If the main verb is a necessity verb—such as täytyy (must), pitää (have to), or kannattaa (should)—the sentence has no grammatical subject in the nominative case. Instead, the person who "must" do something is in the genitive. Because of this structure, the complete object takes the nominative case (the basic dictionary form) without the -n suffix.
The Negative Exception
As always in Finnish, if the sentence is negative, the object must be in the partitive case. This rule overrules any verb chain rules. Even if the sentence uses a necessity verb that normally triggers a nominative object, making it negative requires the partitive.
Inherent Partitive Verbs
Some verbs, like pelata (to play a game), naturally demand a partitive object. When such a verb appears in a verb chain, its object remains in the partitive case, completely ignoring the necessity or standard verb at the start of the chain.