Nominative Case / प्रथमा विभक्ति (prathamaa vibhakti): Nominative Case or प्रथमा विभक्ति (prathamaa vibhakti) 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. 

Study the following sentences. These are in nominative case. In the previous chapters we have already come across these sentence formations. These are mentioned again for completness.

In sentence 1 above, if we ask the question "who is going?", the answer that comes is "boy". So, boy is the subject in the sentence and the noun-form is in nominative case or prathamaa vibhakti. Similarly in sentence 8, if we ask the question "what is falling?", the answer that comes is "fruit". In this fruit is the subject. 

So, answer to the question "who" or "what" is the subject of the sentence and is always in nominative case. The verb form follows the number or वचन (vachana) of the noun. The followings are few rules where the nominative case must be used as the noun-form.