मितां ह्रस्वः

Operational rule vidhi

The उपधा letter of a मित्संज्ञक अङ्ग becomes ह्रस्व when followed by the णिच् प्रत्यय.

Word by word (padaccheda)

Target मिताम् ṣaṣṭhī · bahuvacanaSubstitute / term prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

मिताम्

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

As a rewrite rule (sketch from case endings)

मिताम् →

Replace mitām with hrasvaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The roots having an indicatory म् retain their penultimate short vowel before the causative णि।
The मित् roots are घटादि a subdivision of Bhwadi (800 to 873), and all other roots that end in अम् as शम् and तम् of Divadi class. Thus घटयति, व्यथयति, जनयति, रजयति, शमयति, ज्ञपयति ॥ Some read the anuvritti of the word 'optionally' from the last sutra into this. This will then be a limited option only (vyavasthita-vibhasha). The forms उत्क्रामयति and संक्रामयति are thus explained.

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