Genitive
The ending -y has come to be applied to all feminine nouns.
The ending -a has come to be applied to all neuter nouns and to the majority of masculine nouns. In the 16th century, ending -å, in other words non-acute -a, was applied to neuter stems ending in -ьje: oká (of an eye) but wesela (of a wedding).
After a period of frequent changes (which still has not ended) the ending -u is still applied to some masculine non-personal nouns, but there is no major connection with the original form.
Until the 16th century, the ending -ej, used in the declension of pronouns, was applied to all nouns in genitive and dative and to feminine nouns in the locative if a noun had an old stem ending in -ja-: paniej, rolej, duszej (lord, role, soul).
Read more about this topic: History Of Polish, Changes in Grammar, Declension, Nouns