न हास्तिनफलकमार्देयाः

Operational rule vidhi

But when the words 1. हास्तिन 2. फलक and 3. मार्देय precede पुर , the acute accent (उदात्त) does not fall on their final.

Word by word (padaccheda)

Indeclinable न avyayaSubstitute / term हास्तिन- फलक- मार्देयाः prathamā · bahuvacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

न हास्तिन-फलक-मार्देयाः

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

Commentary

But when the words 1. हास्तिन 2. फलक and 3. मार्देय precede पुर , the acute accent (उदात्त) does not fall on their final.
This is an exception to 6.2.99. Thus हास्तिनपुरम्, फलकपुरम्, मार्देयपुरम् ॥ The son of मृदु is मार्देय formed by ढक्, the word belonging to Subhradi class. The उ is elided by 6.4.147.

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