ḍā , rau and ras are substituted respectively in the room of the three-affixes of the 3rd Person of luṭ (First Future), both in the parasmaipada and ātmanepada ,
Thus in the Parasmaipada कर्ता 'he will do,' कर्तारौ 'they two will do;' कर्तारः 'they will do.' So also in the Atmanepada we have अध्येता, अध्येतारौ, अध्येतारः ।
Why do we say 'in the 3rd person'? Observe श्वः कर्तासि । श्वोऽध्येतासे ॥,