Ode to Rēs
-ēs, -em, -eī, -eī, -ē,
-ēs, -ēs, -ērum, -ēbus (x2)
These fifth declension endings help you form the Latin noun for ‘thing’:

Rēs, rem, reī, reī, rē,
Rēs, rēs, rērum, rēbus (x2):
This fifth declension noun means “fact” or “thing” and it is feminine.