आदेच उपदेशेऽशिति

In a root, which in the system of grammatical instruction (i.e. the धातुपाठ), ends with a diphthong (ए , ऐ , ओ, औ), there is the substitution of आ for the diphthong, provided that no affix with an indicatory श् follows it.
The word धातोः is to be read into this sutra from 6.1.8. Thus ग्लौग्लाता, ग्लातुम्, ग्लातव्यम्, शौ - निशाता, निशातुम्, निशातव्यम् ॥ Why do we say ending with an एच् (diphthong)? Observe कर्त्ता from कृ, and हर्त्ता from हृ ॥ Why do we say in upadesa or Dhatupatha ? Observe चेता, स्तोता where चे and स्तो are not the forms taught in the first enunciation of the root in the Dhatupatha, but are secondary forms. Why do we say, before non-शित् affixes? Observe ग्लै + शप् + तिप् = ग्लायति, म्लायति ॥ Exception is, however, made in the case of the affix एश् of the Perfect: as जग्ले, मम्ले ॥ This is explained by interpreting the word शित् as शिदादि, that is the affixes having an indicatory श in the beginning: एश् has श् at the end. This is done on the maxim यस्मिन् विधिस्तदादावल् ग्रहणे "when a term which denotes a letter is exhibited in a rule in the form of the Locative case, and qualifies some thing else which likewise stands in the Locative case, that which is qualified by it must be regarded as beginning with the letter which is denoted by the term in question and not as ending with it".

The word अशिति is an example of प्रसज्यप्रतिषेधः. a simple prohibition of a contingent case. Therefore, the substitution of आ for एच् vowels, is not caused by the affixes that follow, but must take place prior to the occasion for the application of the affixes arises. Thus 3.1.136 ordains क after roots ending in आ; so that ग्लै and म्लै are presupposed to end in आ when applying this affix: thus सुग्लः, सुम्लः ॥ Similarly by 3.3.128 we apply युच् to ग्लै and म्लै by presupposing it as आदन्त roots: as सुग्लानः, सुम्लानः ॥

The word आ is understood in all the subsequent sutras upto 6.1.57.