āyan for 1. ph 2. ey for ḍh 3. īn for kh 4. īy for ch and iy for gha are substituted when these consonants stand in the beginning of an affix.,
Thus फक् 4.1.99 = आयन, as नड + फक् = नाडायनः, चारायणः ॥ ढक् 4.1.120 = एय as सौपर्णेयः, वैनतेयः ॥ ख 4.1.139 ईन, as आढ्यकुलीनः, श्रोत्रियकुलीनः ॥ छ 4.2.114 = ईय, as गार्गीयः, वात्सीयः ॥ घ 4.1.138 = इय, as क्षत्रियः ॥
Why do we say 'of an affix'? Observe फक्कति, ढौकते, खनति, छिनति and घूर्णते, where these consonants are in the beginning of a root. Why do we say 'in the beginning'? Observe उरुदघ्नम् जानुदघ्नम्, where घ is in the middle of the affix; and remains consequently unchanged.
These आयन &c, substitutions should be understood to have taken place at the very time the affixes फक् &c, are taught, and these substitutions being made, the rule of accent, which makes an affix acute on the first syllable, applies. Thus ख is not acute, but ई of ईन् substituted for it. Similarly in 4.4.117, the affix taught is घच् with an indicatory च्, showing that the final of the affix इय will be acute 6.1.163, and not of घ, for घ would have been acute by the general rule affix.
In शमेर्ढः (Unadi 1.99), शमेः ख (Unadi I: 102), the ढ and ख remain unchanged, and we have शाण्डः, and शंखः ॥ This is explained on the maxim of उणादयो बहुलम् 3.3.1. In the sutra ऋतेरीयङ् 3.1.29, the affix ईयङ् has been taught and not छङ्, this shows that the rule of substitution herein taught does not apply to the affixes to be added to the verbal roots. Thus एजेः खश् 3.2.28, पदरुजविशस्पृशो घञ् 3.3.16. Here the ख and घ are not be replaced by ईन and इय ॥ In fact, they cannot be regarded as affixes, but only as इत् or servile letters. By 1.3.8, the gutturals are इत् except in Taddhita : therefore, the substitutions take place in Taddhita affixes, where the ख and घ are not इत् ॥
The final म् in आयन् and इन् should not, however, be considered as servile (इत्) though they be final consonants. This we infer from the sutra प्राचामवृद्धात् फिन् बहुलम् ॥ Here the न् of फिन् is indicatory, showing the position of the accent. But फ is replaced by आयन्, so if the न् of आयन् were also to be indicatory (इत्) there would have been no necessity of adding म् in फिन् ॥ Hence the fact of this न् in फिन् shows that the न् in आयन् and इन् are not इत् ॥,