आच्छीनद्योर्नुम्

When the affix शतृ (अत्-अन्त्) comes after a verbal अङ्ग (stem) ending in अ or आ, it may optionally take the augment नुम् before the Neutral case-ending शी and before the feminine affix ई।
Thus तुदती कुले or तुदन्ती कुले, तुदन्ती ब्राह्मणी, तुदती ब्राह्मणी, याती कुले, यान्ती कुले, याती ब्राह्मणी, यान्ती ब्राह्मणी, करिष्यती कुले, करिष्यन्ती कुले, करिष्यती ब्राह्मणी, करिष्यन्ती ब्राह्मणी ॥ अत्रान्तरङ्गत्वादेकादेशे कृते व्यपवर्गाभावादवर्णान्तादङ्गादुत्तरस्य शतुरिति न युज्यते वक्तुम्, उभयत आश्रये नान्तादिवदित्यन्तादिवद्भायोपि नास्ति भूतपूर्वगत्याश्रयणो वाऽदतो घ्नतीत्येवमादिष्वतिप्रसङ्ग इति, अत्र सभाधिं
केचिदाहुः, शतुरवयवे शतृशब्दो वर्त्तते, अवर्णान्तादङ्गादुत्तरो यः शत्रवयव इति ॥ अपरे पुनराहुः, आदित्येतेन शीनद्यावेव विशेष्यते, अवर्णान्तादङ्गादुत्तरे ये शीनद्या तयोः परतः शत्रन्तस्य नुम्भवतीति, तत्र येन नाव्यवधानं तेन व्यवहितेपि वचनप्रमाण्यादिति तकारेणैव व्यवधानमाश्रयिष्यते ॥ आदिरिति किम्, कुर्वती, सुन्वती ॥ शीनद्योरिति किम्, तुदताम्, नुदताम् ॥

The form तुदती is thus evolved. तुद् + श + शतृ + ङीप् ॥ The vikarana श is added by 3.1.77, & ङीप् by 4.1.6. This is equal to तुद् + अ + अत् + ई = तुदती the अ + अ becoming अ by the rule of पररूप ekadesa 6.1.97. This ekadesa operation being antaranga, now there is no शतृ affix coming after a stem ending in अ, and therefore this sutra will not apply. If you say the ekadesa will be considered as the final of तुद् by 6.1.85, we reply, that the antachvadbhava of that sutra will not apply when simultaneous operations are to be performed, for अ cannot be said at one and the same time as the final of तुद् and the beginning of अन् (शतृ). If you say the maxim सांप्रतिकाभावे भूतपूर्व गतिः (when a word cannot denote something which actually is what is expressed by the word, it must be understood to denote something which formerly was what is expressed by it), will apply here, and that तुद् will be considered to end in अ because it formerly did end in the affix अ of श; we reply that then the present rule should apply to forms like अदती घ्नती &c. For here also we have अद् + शप् लोप + शतृ + ङीप्; and अद् should be considered to end in अ, because it had the affix शप् after it, though it was afterwards elided. The maxim quoted, therefore, proves too much.

To solve these objections, some say, that the word शतृ in this sutra means 'a portion of the affix शतृ, such as त्' and the sutra means 'after a stem ending in अ, to the portion त् of the affix शतृ there is added नुम्.' Others say, the word आत् in the sutra qualifies शी and नदी and not शतृ; and the sutra means "नुम् is added to a stem ending in शतृ, when शी or नदी affixes follow after a stem ending in अ." Therefore in तुदती the affix ई is considered to come after the अ of तुद, the intervening त् not debarring it.

Why do we say "after अ"? Observe कुर्वती and सुन्वती, no optional नुम् is added here. Why do we say "शी and नदी following"? Observe तुदताम् and नुदताम्