(Whitney §267-268)
The first (प्रथमा) of the eight cases is the Nominative. This case is used to name the subject of a sentence and any nouns or adjectives qualifying the subject. This includes predicate nominatives, or objects of a linking (copulative) verb that serve as subject complements, such as the word “man” (पुरुषः) in the sentence ”he is a man” (स पुरुषः).
According to Whitney §268, there are two unusual uses of the nominative. First, with middle verb-forms meaning “to consider” with regard to oneself, whatever one considers oneself to be is in the nominative. Thus:
स मन्येत पुराणवित्।
“He considers himself wise in ancient things.”
Secondly, any word quoted by the use of इति is always in the nominative, even if the word it is qualifying is in another case, such as:
ताम अग्निष्टोम इत्याचाक्षते।
“They call that ‘agniṣṭoma’.”
Nominatives are not used when a person is addressed in speech. This is the role of the vocative, though often (but not always) the vocative and nominative forms of a noun are the same.