"Yesterday I napped for one hour."
Translation:Hieraŭ mi dormetis unu horon.
'dum unu horo' is ok, but 'por unu horo' is not.
... because 'for' in English has multiple meanings and depends on context, but in Esperanto you need to use the specific one, in this case 'dum' which means 'for' in the context of time (i.e. during).