"Where are the graves?"
Translation:Ubi sunt sepulchra?
The ending "-um" tells you it is most likely a word with the neutral gender, and not masculine (endings -us).
At a beginning stage of learning you could see it like this: feminines ending in -a turn to -ae masculines -us are -i neutrals -um turn to -a
Examples for neutral words: Oppidum, oppida. (town) Malum, mala. (apple) Ornamentum, ornaenta. (jewels)
This is by no means a comprehensive explanation, nor paradigm, as there are additional endings other than those 3. For more info, look for 'latin declensions'. :)
Because it's neuter ending in "um".
um -> a
There are several declensions groups in Latin: They are characterized by their nominative ending, and their genitive ending:
Most of the members of this group are feminine nouns.
a -> ae
(a: nominative, ae: genitive)
It's called the group of genitive in ae.
(But as it's your question, we notice also ae = plural)
Example: Rosa, rosae, feminine.
(but some exceptions like Poeta, poetae, masculine)
All are on this model:
Singular : rosa, rosa,rosam, rosae, rosae, rosa.
Plural : rosae, rosae, rosas, rosarum, rosis, rosis.
The model is dominus, domini.
So, genitive with "i", is the name of the 2nd group.
The plural is also with "i".
Sing.: dominus, domine, dominum, domini, domino, domino.
Plur. : domini, domini, dominos, dominorum, dominis, dominis.
There are several subgroups:
Most of them are masculines, but some exceptions exist.
For the neutral gender. um -> a
model: oppidum, oppida
Sing. : oppidum, oppidum, oppidum, oppidi, oppido, oppido.
Plur. : oppida, oppida, oppida, oppidorum, oppidis, oppidis
Note: I used the order Nom., Voc., Acc., Gen., Dat., Abl.