аббат - singular masculine
аббаты - plural

аббат - nominative

аббатя - genitive

аббатю - dative

аббате - accusative

аббатем - instrumental

аббате - prepositional

There are two choices in instrumental ем and ом. What are the rules on declension. There are so many choices to choose from in genitive, dative, accusative, instrumental, and prepositional. Kindly lay down the rules. Thanks.