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3rd declension:  Many nouns belong to the 3rd declension.

The nominative singular is different for each word.  urbs, navis, rex, pater.

All nouns and including 3rd declension nouns are listed in the vocabulary with the nominative singular, genitive singular and gender.  We have not had the genitive yet, but get to know it.  The Latin Scholars have deduced that this is the best way to list vocabulary in the glossary.  The genitive case singular ending is different for each declension so this is the only sure fire way to know which of 5 declension a word belongs to.  For 3rd declension the BASE is also very important.  When declining a 3rd declension noun, use the nominative singular form once and then use the base of the genitive singular for the other forms.

Genitive Plural is sometimes -um and sometimes -ium.  When do you use which ending?  Well there are 3 simple rules.  If any one of these rules fit, then the ending is -ium.

Rule 1:  If the Nominative singular and the genitive singular have the same number of syllables.  Like:  navis, navis;  hostis, hostis;   caedes, caedis
Rule 2:  If the genitive singular BASE (drop -is) ends in double consonants.  Like:  urbs, urbis;  pars, partis;   mons, montis;  hostis, hostis
Rule 3:  If it is a 3rd Declension adjective.

Sometimes the Ablative singular is an -i.    This ending is only used for 3rd declension adjectives:   tristis, omnis, fortis  etc.

Irregular Verbs: