Learn Finnish: Isoäiti leipoo pullaa sunnuntaina. - Grandmother bakes buns on Sunday.

Because it is the object of the sentence and grandmother is baking an unspecified amount of it. Even though 'buns' is countable in English, Finnish often treats baked goods like pulla as an uncountable mass noun when talking about the general activity of baking them.

What case is sunnuntaina, and why is it used here?

It is in the essive case, recognizable by the -na ending. Finnish uses the essive case for days of the week to express that something happens 'on' that specific day. Therefore, sunnuntai becomes sunnuntaina.

The verb is leipoo, but the 'I' form is leivon. Why does the p stay strong here?

The dictionary form is leipoa, which is a Type 1 verb. These verbs usually drop to the weak grade in personal forms (like p changing to v in leivon). However, the third-person singular (he/she/it) always uses the strong grade, keeping the p in leipoo.

Could I move sunnuntaina to the beginning of the sentence?

Absolutely. Finnish word order is quite flexible. Saying Sunnuntaina isoäiti leipoo pullaa is completely natural and just places a slight emphasis on the fact that this happens on Sunday.