3.1.100 गदमदचरयमश्चानुपसर्गे

Word by word (padaccheda)

Left context गद- मद- चर- यमः pañcamī · ekavacanaIndeclinable च avyayaRight context अनुपसर्गे saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

गद-मद-चर-यमः च अनुपसर्गे

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix यत् comes after roots गद् 'to eat', मद् 'to be mad', चर् 'to go, to eat' and यम् 'to restrain', when these roots do not take any preposition.
Thus गद्यम् 'what is to be spoken, i.e prose'; मद्यम् 'wine'; चर्यम् 'what is to be followed'; यम्यम् 'what is to be restrained'.

Why do we say 'when not preceded by a preposition'? When compounded with an upasarga, these roots will take ण्यत्; thus प्रगाद्यम्; प्रमाद्यम् &c.

The root यम् would have taken the affix यत् by sutra 98 also, as it ends in म and is preceded by अ; its separate enumeration in this sutra is in order to show, that the compound verb यम् will not take the affix यत्. Therefore with regard to यम्, this is a niyama or restrictive rule and not a vidhi or original rule.

Vart:- The root चर when preceded by the preposition आङ् takes the यत्, affix when the sense is not that of a preceptor; thus आचर्यो देशः 'an approachable or communicable country'. But when the sense is that of a teacher the root takes ण्यत्; thus आचार्यः 'a preceptor'

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.