The affix aṇ -- prāgdīvyato'ṇ [[4.1.83]] comes after a word denoting 'some subject of study' in the sense of 'who has studied that or who understands that'.,
In others words, the sutra means that the affix is added to a word in the accusative case in construction, denoting a person who studies or knows, that. As छन्दोऽधीते = छान्दसः 'who studies Prosody'. वैयाकरणः 7.3.3 'who knows grammar'. So also नैरुक्तः, नैमित्तः 'who knows the causes'. भौहूर्त्तः, औत्पातः ॥ The word तद् is repeated in this sutra, in order to show a distinction between one who studies and who understands.,