Rule. A taddhita affix is added, optionally, after the first among the samartha words. The three words samarthānām, prathamāt and vā are a heading (adhikāra). They are read into every taddhita rule as far as prāgdiśo vibhaktiḥ (A 5.3.1, SK 1947).
What the three words mean:
- samartha, "capable": a word that is fully settled, that is, one on which the sandhi operations have already been performed. Bhaṭṭoji's commentators explain why this gloss is needed. The other sense of "capable", being joined in one meaning, is already supplied for every word-rule by the paribhāṣā samarthaḥ padavidhiḥ (A 2.1.1). A paribhāṣā is a rule about how other rules are read. So the word would be idle in that sense. A word is capable of conveying its meaning only once its sandhi is done.
- prathamāt, "after the first": after the word named first in each sense-rule. In the rule tasyāpatyam, "his offspring", that word is tasya, "his" — the genitive.
- vā, "optionally": the affix need not be used, so the phrase with separate words,
aśvapater apatyam, stays correct.
Bhaṭṭoji's commentators add two points. Samarthānām is a partitive (nirdhāraṇa) genitive: "among the samartha words". And the sūtra is a heading, not an operative rule (vidhi), because it names nothing to be added.
Example.
aśvapater apatyam "the offspring of aśvapati" → the sense-rule tasyāpatyam names the genitive word first → aśvapateḥ, a finished word, is the samartha → the affix goes after it: aśvapati + aṇ (SK 1073–1076) → āśvapatam.
By vā, the phrase aśvapater apatyam is also correct.
Watch out. The affix goes after the first-named word of the sense-rule, not after the word for the new sense. In aśvapater apatyam the affix follows aśvapati; apatya "offspring" is the meaning the affix expresses.
These three words [samarthānām, prathamāt, vā] are made governing (adhikāra) — that is, as far as prāgdiśaḥ [the sūtra 'prāgdiśo vibhaktiḥ' (A 5.3.1)]. Sāmarthya [here] is the state of being fully settled (pariniṣṭhitatva), that is, the state of having had the sandhi operations performed.
- commentary BM: since no vidheya appears, this is not an independent rule but an adhikāra (by the svarita-pratijñā); the upper limit 'prāgdiśaḥ' is the sūtra 'prāgdiśo vibhaktiḥ' (5.3.1) — hence the bracketed identification. samarthānām is a nirdhāraṇa genitive: 'after that one among the samartha words which is mentioned first in each sūtra such as tasyāpatyam'. BM adds that 'samarthāt prathamād vā' would have been the easier wording (TB agrees), and reports that some take the plural to restrict the adhikāra to cases where several samartha words combine, so that it does not reach svārthika rules such as prāgdiśaḥ. Followed for 'first among the samartha words'.
- commentary BM: the gloss 'pariniṣṭhitatva' answers the objection that, since taddhitas are taught after subantas, sāmarthya as ekārthībhāva is already secured by the paribhāṣā 'samarthaḥ padavidhiḥ' (2.1.1) and the word would be idle; samartha = paṭu = śakta, and a word is 'capable' of its effect (conveying meaning) only once the sandhi operations are done — hence 'kṛtasandhikāryatva'. TB: the fruit of this will become clear in the kaumudī itself under 'tasyāpatyam'. Followed for the rendering of sāmarthya.
- textual The source reads 'prādgiśa-' (with a dangling hyphen) where the sense requires prāgdiśaḥ; BM's pratīka 'prāgdiśa iti yāvad iti' and its identification of 'prāgdiśo vibhaktiḥ' as the upper limit confirm this. The transmitted reading is left in the source; translated as prāgdiśaḥ.
- textual The second half of the TB block filed here ('mahotsargān āha — prāgdīvyata ityādinā …', on the word dīvyat) comments on the next sūtra (SK 1073); its content is recorded there.