The nominative case
The nominative case is a naming case. Its most frequent function is to indicate the subject of a finite verb.
The subject of a verb will match the person and number of the verb form: a singular noun in the nominative case will have a third person singular verb form, and a nominative plural noun acting as the subject will match a third person plural verb. We use the term agreement to refer to this matching of grammatical properties.
Consider these examples. In his account of Theseus’ adventures, Hyginus says that Theseus came to Crete:
Theseus uenit
The dictionary form for Theseus’ name in Latin is “Theseus, Thesei, m,” so Theseus is nominative, singular, and masculine. uenit, “he came,” is perfect, active and indicative; since it is in the third person singular, it agrees with the subject Theseus.
In the story of the seven heroes who fought against Thebes, Hyginus says, “The seven leaders were going there” (in order to fight),
Septem ductores ibant
ductor, ductōris, m., means “leader, commander. The form ductores is masculine, nominative and plural. (We’ll see the plural forms of the nominative case below.) ibant is in from the irregular verb eō, “to go.” It is imperfect indicative active. Even in the irregular form, you can recognize that the ending is third person plural, so it agrees with the plural subject ductores.