अचस्तास्वत् थल्यनिटो नित्यम्

After a root which ends in a vowel and after the Periphrastic Future affix तासि is always devoid of the इट् augment (after such a root) थल् the personal ending of the लिट् (Perfect Tense) तास्, does not also get the augment इट् ।
Those vowel-ending roots in Dhatupatha which are invariably anit before the affix तास् (Periphrastic Future), are also anit before the Perfect ending थल् ॥ As याता (Periphrastic Future), ययाथ (Periphrastic), चेता, चिचेथ, नेता, निनेथ; होता जुहोथ ॥ Why 'ending in a vowel'? Observe भत्ता, बिभेदिथ ॥ Why 'like तास्'? Observe लूत्वा but लुलविथ, Why 'थल'? Observe याता but ययिव, ययिम ॥ The word नित्यं qualifies अनिट्, if therefore तास् be optionally anit, then the थल् will be सेट् always. As तास् — विधोता or विधविता; the थल् is विदुधविथ ॥

Here इट् is optional before तास् by 7.2.44. Similarly चक्रमिथ, for क्रम् is anit in atmanepada, and सेट् in Parasmaipada. In fact, whereever there is want of इट्, whether by complete prohibition or optional prohibition, all that is regulated by 7.2.13: this is the view adopted by the Kasika. But another view is that the prohibition of this sutra applies to the इट् of 7.2.13 only, but does not debar the option of 7.2.44; so they have two forms विदुधोथ and विदुधविथ ॥ This view is applied by them to 7.2.46, as सस्वर्थ or सस्वरिथ 7.2.44.

Why have we used the word वत् in तास्वत् ? The force of वत् is that the root should have a form in तास् and then be anit; and if a root has no Periphrastic Future form, and thus its तास् is anit, such a root is not governed by this sutra. As जघसिथ and उवयिथ ॥ Here the roots घस् and वय् the substitutes अद् and वेञ् respectively have no Periphrastic Future form, and hence this rule does not apply to them.