Can anyone explain the ways singular nouns change forms at the end and why? I feel like an explanation of why they change the way they do would be helpful for me.
I don’t think there’s an answer to your ‘why’ but maybe you are looking for a rule to follow, rather than a reason?
Such things develop over millennia and there’s no way to answer ‘why’ any more than there’s an answer to - why do we pluralise nouns in English by adding an ‘s’!
My suggestion would be to review the lessons where the -ean and other plural endings begin and simply write them down beside the singular form.
The patterns will soon make more sense and become more familiar...
Good luck :)