Grandpa actually means isoisä and the word vaari is something we use like in every day speech. Vaari is kind of a pampering name and thus grandpa could be also pappa or ukki. Isoisä is the actual word. Using the word vaari is very situational and my other grandpa was vaari and the other one was ukki but both of them were isoisä.
Again, the partitive case object indicates an event in progress and not completed and requires present progressive in the English translation. Grandpa paints the roof would be vaari maalaa katon. Accusative case object. While every English verb can be grammatically translated in either simple present or progressive, standing alone, an object determines the correct usage in translation from Finnish.
In many languages "he does paint" would also be a valid translation of the stand alone subject and verb.
The reason why "grandpa paints the roof" is incorrect is that roif, kattoa, is in partitive case. Use of partitive is an extremely important part of learning Finnish, and part of the goal of tje latter exercises in the second tree.
My suggestion is, whenever the object is partitive, ask yourself why--"miksi partatiivi?" Here the object is in partitive.
Is roof a mass noun? No, obviously it's countable.
Is roof preceded by a number other than one? No.
Is maalata a partitive verb, ie, irresultive like etsiä, or expressive of an emotion like love, hope, belief, etc.? No.
The other common reason why an object is partitive is that the action of the verb is in progress, not completed or a future/regular event, which is the case here. That's why the object is kattoa instead of katto or katon. So the correct translation is, "is painting" and not "paints".
That would be vaari maalaa katon. Accusative case object. Partitive indicates an event in progress and incompetent, so present progressive is required in the English translation. I don't know if Duo has a search function to find then, but this has been addressed multiple times during the course. ;-)
In my opinion, both should be accepted as I recall this was OK in the more established courses such as English-to-Spanish. Grandfather/grandpa, Isoisä/vaari just has the difference between the formal name and the familiar.
Also several other common variations of "grandpa" should be accepted, such as "ukki", "pappa" etc.