न भूसुधियोः

Operational rule vidhi

The words भू and सुधी do not undergo यणादेश when followed by अजादि सुप् प्रत्यय.

Word by word (padaccheda)

Indeclinable न avyayaTarget भू- सुधियोः ṣaṣṭhī · dvivacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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

न भू-सुधियोः

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

Commentary

The semi-vowel substitution does not take place in the case of अङ्ग (stems) ending in भू or the word सुधी before affixes beginning with a vowel.
Thus प्रतिभु - प्रतिभुवौ, प्रतिभुवः; सुधी - सुधियौ, सुधियः ॥ 6.4.77.

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