भियोऽन्यतरस्याम्

Operational rule vidhi

The last letter of the अङ्ग of verb root 'भी' is optionally converted to ह्रस्व-इकार when followed by हलादि-सार्वधातुक कित्/ङित्-प्रत्यय.

Word by word (padaccheda)

Target भियः ṣaṣṭhī · ekavacanaRight context अन्यतरस्याम् saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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

भियः अन्यतरस्याम्

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

Commentary

इ is substituted for the ई of भी before a सार्वधातुक affix beginning with a consonant and having an indicatory क् or ङ्।
Thus बिभितः or बिभीतः, बिभिथः or बिभीथः, बिभिवः or बिभीवः, बिभिमः or बिभीमः ॥ But बिभ्यति before a vowel-beginning affix and बिभेति before a non-ङित् affix and भीयते before an ardhadhatuka affix.

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