धातोस्तन्निमित्तस्यैव

For the final dipthongs ओ and औ of a root, are substituted अव् and आव् respectively, before an affix beginning with य् , then only when such diphthong has been itself first evolved by that affix.
The words एचः, वान्तोयि प्रत्यये are understood in this sutra. The word तन्निमित्त means 'caused by that' i. e. caused or occasioned by that affix beginning with य ॥ Thus लू forms its Future Passive Participle by यत् 3.1.97, this affix causes the guna of ऊ by 7.3.84: Thus लू + य = लो + य, which according to the present sutra becomes लव्यम् ॥ So also पू + य = पव्यम् ॥ लू + ण्यत् 3.1.125 = लौ + य = अवश्य लाव्यम् and अवश्य पाव्यम् ॥ Why do we say 'of a root'? This rule should not apply to a nominal stem. For then, though it may be all right in the case of बभ्रु + य = बाभ्रो + य = बाभ्रव्यः; it will not apply to cases like गो + य = गव्य, नौ + य = नाव्य, where ओ and औ are not caused by the affix, but are integral parts of the stem before the affixes were added. Why do we say 'caused by that affix itself'? The substitution will not take place, when the change is not caused by that affix. Thus the Passive of वे with the upasarga आ is आ + वे + यक् + ते = Here by Samprasarana 6.1.15, वे becomes उ; as आ + उ + य + ते; now by sandhi आ + उ = ओ 6.1.87, we have ओ + यते = ओयते ॥ Since ओ is not caused by य, there is no अव् substitution. So also औयत, लौयमानिः, पौयमानिः 4.1.95. The word एव in the aphorism has the force of limitation, with regard to roots. In the case of roots, ओ and औ before य are changed then only to अव् and आव् when य has caused the production of ओ and औ; in case of nouns there is no such limitation. Here the substitution takes place whether the य has caused the production of ओ and औ or not.