What is Prathamā Vibhakti

प्रथमा विभक्ति or Nominative Case represents the noun-form to represent the "who" or "what" on the verb in the sentence.
In other words, nominative case represents the subject or कर्ता (kartaa) of the sentence.

Let us look at the following sentences to understand this.

Prathama Vibhakti sentences in Sanskrit

1. कविः लिखति (Poet writes)
In this sentence, कविः is the subject of the actively used verb in the sentence.

2. फलं पतति (Fruit falls)
In the above sentence, फलं is the subject of the verb पतति.

In sentence 1 above, if we ask the question, 'who writes?' the answer that we get is 'Poet writes'.
In sentence 2 above, if we ask the question, 'what falls?' the answer that we get is 'Fruit falls'.
So, in general, answer to the question 'who' or 'what' is the subject of the sentence and is always in nominative case.

Examples of Prathamā Vibhakti in Sanskrit

Prathamā Vibhakti of Ram

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	रामः	रामौ	रामाः

Prathamā Vibhakti of Lataa

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	लता	लते	लताः

Prathamā Vibhakti of Fal (Phal)

विभक्ति	एकवचन	द्विवचन	बहुवचन
प्रथमा	फलम्	फले	फलानि