Learn Italian: I manubri si devono rimettere sulla mensola. - The dumbbells must be put back on the shelf.

Because we are using the si passivante (passive 'si'). In this construction, the object being acted upon, i manubri (the dumbbells), becomes the grammatical subject. Since manubri is plural, the modal verb dovere must also be conjugated in the plural form (devono).

Can I attach the si to the end of rimettere instead of putting it before devono?

Yes, you can! When you have a modal verb (dovere, potere, volere) followed by an infinitive, you can either put the pronoun before the conjugated verb (si devono rimettere) or attach it to the end of the infinitive by dropping the final 'e' (devono rimettersi). Both are perfectly natural and mean the exact same thing.

Could this sentence also translate as 'One must put the dumbbells back'?

Absolutely. The si passivante is very closely related to the impersonal si (meaning 'one', 'we', or 'people in general'). Whether you translate it as passive ('The dumbbells must be put back') or impersonal ('One must put the dumbbells back'), the meaning is exactly the same. Just remember that the verb still has to be plural to agree with i manubri.

What makes up the word sulla?