For the m of the affix mat is substituted va if the aṅga (stem) ends in m or a ( and ā) or if these are in the penultimate position; but not after yava etc.,
After a stem ending in म् or having म् as its penultimate letter, and after a stem ending in अ or आ, or having these letters as its penultimate, there is substituted व् for the म् of मतुप् ॥ First after stems ending in म्, as किंवत् (किंवान्), शंवत् (शंवान्). Secondly म् penultimate : as शमीवान्, दाडिमीवान् ॥ Thirdly a stem ending in अ or आ, as वृक्षवान्, प्लक्षवान्, खट्वावान्, मालावान् ॥ Fourthly अ or आ in the penultimate : as- पयस्वान्, यशस्वान्, भास्वान् ॥
Why do we say \म् or अ ending or म or अ penultimate\? Observe अग्निमान्, वायुमान् ॥ Why do we say \with the exception of यव &c\? Observe यवमान्, दल्मिमान्, उर्म्मिमान् ॥
The following is the list of यवादि words.
1. यव, 2. दल्मि, 3. उर्म्मि, 4. भूमि, 5. कृपि, 6.कुञ्चा, 7. वशा, 8. द्राक्षा, 9. घ्राक्षा ॥ These words either end in म and अ or have these as their penultimate. 10. ध्रजि, 11. ध्वजि, 12. सञ्जि. These are exceptions to 8.2.15. 13. हरित् 14. ककुत्, 15. गरुत् ॥ These are exceptions to 8.2.10. 16. इक्षु, I7. मधु, 18. द्रुम, 19. मण्ड, 20. धुम ॥ These are exceptions to 8.2.11.
This is an akritigana. Wherever in a word, the म् of मतुप् is not changed to व्, though the rules require it, that word should be classified under यवादि class. In the secondary word नार्मतं (= नृमत इदं), the व् change has not taken place, because the अ is here a Bahiranga, the real vowel being ऋ ॥
The word मात् in the sutra is the Ablative of म, i.e. of म् + अ; it is a Samahara Dvandva of these two letters.,