न विभक्तौ तुस्माः

The final dental consonants and the final स् and म् are not इत् , in affixes called विभक्ति or inflective affixes.
This is an exception to the previous sutra which declared all final consonants to be इत्. In case terminations, the final त्, थ्, द्, ध्, न्, स्, and म्, are not इत्. Thus the case termination of nominative plural is जस्. Here the final स्, though a consonant is not an इत्; and is not rejected, as in ब्राह्मणाः ॥ The word तु in the sutra means the letters of the class त by virtue of sutra 69 Chapter I.(see ante). The word vibhakti will be defined later on in sutra 1.4.104. These are terminations applied to nouns in declining them through various cases; and to verbs in conjugating them. Thus in sutra 7.1.12, (टाङसिङसामिनात्स्याः) the vibhaktis इन्, आत् and स्य replace टा, ङसि and ङस्. Here the final त् of the affix आत् is not indicatory and hence not rejected, as वृक्षात्. Similarly verbal terminations तस् and थस् as पचतः, पचथः ॥ So also the final म् as ताम् and तम् in अपचताम् and अपचतम् ॥

This exception applies only to vibhaktis and not to affixes in general. Thus the finals of यत् (III.1.97 अचोयत्;) युस् (5.2.123, ऊर्णाया युस्) and श्रम् (3.1.78 रुधादिभ्य श्रम् ।) are इत् and are rejected. This exception, however, does not apply to the vibhakti अत् (in 5.3.12 किमोऽत् or 3.4.106, इटोऽत्). In those two sutras the final त् of the vibhakti is इत्. The reason of this is, that the present sutra is an anitya sutra, i. e. a rule not of general application. In sutra 5.3.24, (इदमस्थमुः) we find, 'the word इदम् takes the vibhakti थमु in the sense of mode or manner.' As इद् + थमु = इत्थम्, 'in this manner.' Now थमु is a vibhakti by 5.3.1. (प्राग्दिशो विभक्तिः), the उ is इत्, the real vibhakti is थम्. Now had the present sutra been of universal application, then there would have been no necessity of adding this उ as indicatory letter, which has been evidently added to save the म् of थम् from becoming an इत्. This sutra is consequently an antiya sutra.