For the final of tyad etc. there is substituted a when an affix called vibhakti follows.,
Thus त्यद् - त्यः, त्यौ, त्ये ॥ तद् — सः, तौ, ते; यद् — यः, यौ, ये; एतद् — एषः, एतौ, एते; एदम् — अयम्, इमौ, इमे; अदस् — असौ, अमू, अमी; द्वि, द्वौ, द्वाभ्याम् ॥ The tyadadi words extend upto द्वि, in the list of the pronouns, for the purposes of this अ substitution. Therefore, the substitution does not take place here in भवत् which forms भवान् ॥ When the word त्यद् &c are employed as names, or as a secondary member in a compound, the substitution does not take place, as त्यद्, त्यदौ, त्यदः, अतित्यद्, अतित्यदौ, अतित्यदः See also (l. 1. 27) commentary. But when they form the principal member of a compound, the substitution takes place, as परमसः, परमतौ, परमते ॥ For case affix (vibhakti) see 5.3.1 also. Those affixes are also called vibhakti and cause these substitutions.,