ya is substituted for the final of yuṣmad and asmad before a case-ending which is not a substitute and which begins with a vowel.,
Thus त्वया, मया, त्वयि, मयि, युवयोः, आवयोः ॥ Why do we say 'beginning with a vowel'? Observe युवाभ्याम्, आवाभ्याम् ॥ If in the sutra 7.2.86, we read the anuvritti of हलि, we need not use अचि in the present sutra. For then this sutra will be a general (utsarga) aphorism, ordaining य before all non-substitute case-endings; and the sutra 7.2.86, will be considered an exception (apavada) to this, with regard to those case-endings which begin with a consonant, where आ will be ordained. In this view of the case, the use of अचि here is explanatory, Why 'when the ending is not a substitute'? Observe त्वद् गच्छति, मद् गच्छति ॥,