The first form, e.g., cena, is the lemma or lexical form. This is the form you use when looking up words in a dictionary or lexicon.
The second form is the lemma declined in the genitive case, singular number. In this instance, it is cenae.
Robert J. Henle wrote,1
All Latin nouns are divided into five main groups called declensions, and in these groups we have a model which shows us the proper endings to use. All we have to find about a new noun is this: What declension does it belong to? This we can tell from the Latin case called the GENITIVE. The GENITIVE SINGULAR always tells to what declension a noun belongs.
Furthermore, the genitive singular declension is important because by removing the case ending, we can determine the word’s stem. For example, the case ending for the first declension noun in the genitive case, singular number is -ae. By removing that, we can determine the stem, which is cen-.
Henle, Robert J. Latin: First Year. Chicago: Loyola Press, 1958.
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