What are you referring to? There could be many cases whereby a word ends in "a" or "e".
en kat - a cat katten - the cat katte - cats kattene - the cats
et navn - a name navnet - the name navne - names navnene - the names
as you can see it's pretty simple: if a word is an en word (en bog), to make it a definite you just add -en at the end (bogen), and if it's an et word (et barn) - add -et at the end (barnet)
to the plurals - this might be more complicated than English, where u just put -s, -es or -ies. In Danish different words require different plural form endings. • most of the words end with -(e)r (en bog - bøger; en pige - piger). • the second most popular plural words ending is -e (en dreng - drenge; et navn - navne) • some words don't have a plural form, they have the same form as their singular form (en mus - mus; et æg - æg)
(bare in mind some words change their letters when making the plural form: et barn - børn; en bog - bøger; en mand - mænd. I don't know a rule for those, you just have to remember them)
and the definites of plural nouns: it's very easy as well, you just have to puy -ne at the end of the plural form of the noun, no matter if it's an en word or an et word (ræve - rævene; æg - æggene; bjørne - bjørnene)
I hope this makes sense and it will help you :)