Learn Finnish: He haluavat laajentaa autotalliaan. - They want to expand their garage.

In Finnish, when the third-person owner is the same as the subject of the sentence (here, he), you must drop the genitive pronoun (hänen or heidän) and only use the possessive suffix on the noun. If you explicitly added heidän here, it would mean they want to expand someone else's garage.

Why is the word for garage in the partitive case (autotallia) before the possessive suffix is added?

The verb laajentaa (to expand) generally takes a partitive object because the action is viewed as a continuous process or a change in degree, rather than a discrete, completed whole. So autotalli becomes autotallia, and then the possessive suffix is attached.

The third-person possessive suffix is often -nsa or -nsä. Why is it -an in autotalliaan?

The third-person possessive suffix has two forms: -nsa/-nsä and -Vn (where V is a copy of the preceding vowel). When the inflected word ends in a short vowel, like the partitive autotallia, it is much more common and natural to use the -Vn form, making it autotalliaan. Both autotalliaan and autotalliansa are grammatically correct, but the first is overwhelmingly preferred in standard Finnish.

Does the coreference rule still apply even though is the object of the infinitive , not the main verb ?