निशाप्रदोषाभ्यां च

Operational rule vidhi

The affix ठञ् comes optionally comes in the remaining senses after the words 1. निशा and 2. प्रदोष .

Word by word (padaccheda)

Left context निशा- प्रदोषाभ्याम् pañcamī · dvivacanaIndeclinable च avyaya

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".

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Full reading

निशा-प्रदोषाभ्याम् च

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

Commentary

The affix ठञ् comes optionally comes in the remaining senses after the words 1. निशा and 2. प्रदोष ।
This makes optional, where by sutra 4.3.11 ante, the ठञ् would have been obligatory. Thus नैशम् or नैशिकम् 'nocturnal'. प्रादोषम् or प्रादोषिकम् ॥

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