3rd declension nouns
As always, look at the dictionary entry for the noun. mercator, mercatoris m merchant nominative genitive gender definition
To find the stem of the noun, go to the genitive form and take off the genitive stem. mercatoris
Add the declension endings to the stem. Nominative S mercator Genitive mercatoris Dative mercatorī Accusative mercatorem Ablative mercatore Nominative Pl mercatorēs Genitive mercatorum Dative mercatoribus Accusative mercatorēs Ablative mercatoribus
Watch out for words with stem changes. rex, regis m king stem = regis
Add the declension endings to the stem. Nominative S rex Genitive regis Dative regī Accusative regem Ablative rege Nominative Pl regēs Genitive regum Dative regibus Accusative regēs Ablative regibus