- (sinä) siivoat -> siivoatko (sinä) = you in singular
- (te) siivoatte -> siivoatteko (te) = you in plural
- he siivoavat -> siivoavatko he = they
Remember that you can't leave out the pronoun in the 3rd person singular nor plural. If Duolingo does not accept sinä or te in sentences like these, you can report it because it should be accepted as there's no way telling from the English sentence which one Duolingo wants as correct.
The verb stem is shorter than the infinitive form - so the infinitive is siivota but the stem is actually siivo- and all the suffixes will be added to that. "Siivo" actually also means "tidiness" or "cleanliness" (ironically tho, saying "Täällä on kauhea siivo!" for some reason means "What a mess!")
There's a lot more to the Finnish verb stems than just that so I'll give you a link to the Uusi kielemme website.