Original: It is a mixture of the second and fourth declension in Latin; the third declension are mostly feminine words ending with a soft consonant; the fourth declension are words on - ej, it matches the Latin fifth declension.
Simplified: They are very much like the declensions in Latin: the first declension are all words on - a, almost all of them are feminine; the second declension are mostly masculine and neuter words ending with a consonant.