ङमो ह्रस्वादचि ङमुण्नित्यम्

After a word ending in ङ् ण् or न् which is preceded by a light vowel, the same consonant ङ् ण् or न् is added invariably at the beginning of the next word, which commences with a vowel.
The word ङमः is in the Ablative singular here; and ह्रस्वात् qualifies ङमः and ङम् itself qualifies the word पदस्य understood, and thus there is tadanta-viddhi. Though the word पदस्य 8.1.16 is in the Genitive singular, yet it should be converted here into Ablative singular, because of its connection with ङमः ॥ अचि is in the Locative singular, but should be construed as Genitive singular here : it is exhibited in the 7th case for the sake of brevity, and of the subsequent sutras. ङम् is a pratyahara meaning ङ, ण and न; and so also ङमुट् is a pratyahara containing the three augments ङुट्, णुट् and नुट् ॥

In other words ङ् is augment after ङ्, ण् after ण् and न् after म्; or that these letters are doubled practically. Thus ङुट् is the augment after a word ending in ङ्, as प्रत्यङ्ङास्ते ॥ णुट् is the augment after a word ending in ण्, as वणणास्ते ॥ नुट् is the augment after a word ending in न्, as कुर्वन्नास्ते, कुर्वन्नवोचत्, कृषन्नास्ते, कृषन्नवोचत् ॥

Why do we say 'ending in ङ, ण or न'? See त्वमास्ते ॥ Why do we say 'preceded by a light vowel'? Observe प्राङास्ते, भवानास्ते ॥ Why do we say "followed by a vowel"? Observe प्रत्यङ्करोति ॥

The Mahabhashya thus comments on this aphorism :-

Vart:- ङमुटि पदादिग्रहणम् "The ङमुट् augment is added to a vowel which stands at the beginning of a word". So that in दण्डिन् + आ (Instrumental Singular), नुट् is not added to आ, because it is not the beginning of a Pada. Then should this vartika be held to be necessary? No, because the word पदात् is understood here: so नुट् will not come in दण्डिना ॥ But then it will come in परमदण्डिन् + आ ॥ Because it is a compound of two nouns, and though the case-affixes have been elided, yet दण्डिन् is here a Pada by reason of pratyaya lakshana; and hence there should be नुट् here added to आ ॥ Answer:- This is no valid objection. दण्डिन् is not here a Pada, on the maxim उत्तरपदत्वे चापदादिविधौलुमता लुप्ते प्रत्ययलक्षणं न भवति "When an affix has been elided by लुक् or लुप्, the pratyaya-lakshana rule will not apply, when the object of it is to give the designation of Pada to the second member of a compound, with the exception of the rule applying to the beginning of a Pada". Thus in परमदण्डिन् the word दण्डिन् is not treated as a पद, because the rule to be applied is to the end here. But the second member of a compound is treated like a pada, when a rule is to be applied to the beginning of a Pada. Thus in दधिसेचौ, the second member सेच् is considered as a Pada for the purposes of the rule सात् पदाद्योः 8.3.111, and the स is not changed to ष ॥ This view proceeds upon the supposition that the word पदात् governs this sutra. But the anuvritti of पदात् ceased with 8.1.27, as we stated before. How are we then to get out of this difficulty? Are we to make the above vartika necessary? No: because the anuvritti of पदे from 8.3.21 runs into this sutra. So that the sutra means अजादौ पदे ङमुड् भवति ॥ "The augment ङमुट् is added to a vowel with which a Pada commences". Not therefore to the case-affix आ in दण्डिना ॥