For third declension singular, some say that it may take either the dative or the ablative form, while others say it takes the dative form. For fourth declension singular, one says the dative forms are used, but the only actual example seems to be the fourth declension word domus whose locative is domi . But domus affords other problems since it was moving from fourth to second declension in classical times, and this form is consistent with its second declension paradigm. From the genitive ending -ae , the noun matella is identified as a first declension noun. Dropping the genitive ending gives the base matell- to which endings are added.
Some nouns have plural forms only.
For these nouns, the principal parts are the nominative and genitive plural forms. Again the genitive identifies the declension, in this case second declension, as well as the base castr- to which endings are added. The boat is anchored to the shore. When the silly clowns visit Rome, the emperor's sense of humor is a surprising sight to behold. Venus, your new hairdo is a pathetic mess .