Whatever has an indicatory u , ṛ and ḷ (with the exception of a root) and the aṅga (stem) ac (añci) get the augment num in the strong cases.,
Thus भवतु has an indicatory उ, formed by the Unadi affix डवतुप्, and it is declined as भवान्, भवन्तौ, भवन्तः ॥ Similarly ईयसुन् - श्रेयान्, श्रेयांसौ, श्रेयांसः, शतृ - पचन्, पचन्तौ, पचन्तः ॥ अञ्च - प्राङ्, प्राञ्चौ, प्राञ्चः ॥
Why do we say 'having an indicatory उक् vowel or the stem अच्'? Observe दृषत्, दृषदौ, दृषदः ॥
Why do we say in strong cases? Observe भवतः पश्य, श्रेयसः पश्य ॥ अञ्च् root is mentioned for the sake of niyama, i.e. of the roots, only अञ्च् gets नुम् and no other root. Therefore उखास्रत्, पर्णध्वत् formed from the roots स्रंसु and ध्वंसु having indicatory उ ॥ (See 3.2.76, 4.1.6, 8.2.72). For the lengthening in भवान् &c, see 6.4.10, 6.4.14: the च् is elided in प्राङ् by 8.2.23, and न् becomes ङ् by 8.2.62: and त् in स्रत् by 8.2.72.
Why do we say 'with the exception of a root' when by the mere fact of including one root अच् all other roots would have been excluded from the scope of this sutra? The specific mention of अधातोः shows that the prohibition applies to original roots, and not to those roots which are derived from nouns. Thus गोमन्तमिच्छति = गोमत्यति formed by क्यच् ॥ Here गोमत्य is a derivative root, in its primitive state it was a noun. The prohibition of अधातोः will not apply to this root and नुम् will be added. Thus गोमत्य + क्विप् = गोमत्य् + ० (the अ is dropped by 6.4.48 = गोमत् + ० (the य् is elided by 6.4.50). Now is added नुम् and we have गोमान् ॥,