दधस्तथोश्च

For the द of दध (the reduplicated form of धा) is substituted ध before the affixes beginning with त, थ, स and before ध्वम्।
The word दध is taken in the sutra as the reduplicated form of धा दधाति and not the root दध धारणे of Bhuadi class, as शप् intervenes there. By the word च we draw in the words स and ध्व ॥ Thus धत्तः, धत्थः, धत्स, धत्स्व, धध्वम् ॥ By the express injunction of this sutra, the elided अ is not considered as sthanivat. The last sutra could not have applied to दध for two reasons. Ist. It does not begin with a बश् letter, for the real reduplicate is धध, and ह is merely a substitute, and is considered asiddha. 2ndly. The form दध does not end in a jhash consonant, but in a vowel अ, and though this अ is elided before these affixes, yet the lopa would be sthanivat. Hence the necessity of च in this sutra. See coutra, the vartika in Mahabhashya.

The word झलि is understood here, and so also झषन्तस्य; and there can be no affix, but begins with त or थ, that can come after दध् ॥ Why do we employ then the words 'before त and थ'? Had we not used these, the sutra would have referred to स and ध्व only, as being in immediate proximity, and the च draws them in. According to Padamanjari the words तथोश्च could have been dispensed with: for before स् and ध्व, the ध change would have taken place by the last sutra, whilst by this sutra, the same change would have taken place before all other झल्-beginning affixes, and such affixes that can come after दध् are त or थ-beginning affixes.

The word jhash is understood here also, therefore the rule applies to दध then only, when it assumes the form of दध्, by the elision of आ; and hence not here दधाति ॥