"We fill the tub and we wash ourselves."
Translation:Labrum implemus et nos lavamus.
It’s a matter of a difference in case (it’s function in the sentence).
Dative = labro, as the indirect object “in labro”.
Nominative = labrum, as the subject “Labrum est”.
Accusative = labrum, as the direct object “Ego labrum impleo”.
Labrum follows the 2nd declension for the various forms of the word.
(Latin expert can verify.)