णत्व Ṇatva Rule, when 'न् n' becomes 'ण् ṇ'
In Sanskrit word formation, the Dental nasal 'न् n' may change to the Retroflex nasal 'ण् ṇ', in certain situations. This is known as णत्व in Sanskrit Grammar.

Note: Retroflex nasal is also called Cerebral nasal.

By 8.4.1 रषाभ्यां नो णः समानपदे and 8.4.2 अट्कुप्वाङ्नुम्व्यवायेऽपि
The dental nasal न् (n) automatically changes into the retroflex nasal ण् (ṇ)
• if it is preceded in the same word by the retroflex sounds ऋ (ṛ), ॠ (ṝ), र् (ra), or ष् (ṣ)
• provided no dental, palatal, or retroflex consonants block the path.

Other Sutras that change 'न्' to 'ण्’

By 8.4.58 अनुस्वारस्य ययि परसवर्णः
• 7.1.58 नुम् changes to Anusvara 8.3.24 ं changes to 8.4.58 परसवर्णः ण् for Roots where the following letter is a Cerebral टवर्गः (ट् ठ् ड् ढ् ण्)
By 8.4.41 ष्टुना ष्टुः
• Consonant Letters स् and Dentals तवर्गः (त् थ् द् ध् न्) when coming in contact with ष् and Cerebrals (ट् ठ् ड् ढ् ण्) , are changed to ष् and cerebral respectively.
 
Devanagari Script Mapping for Nasals

Devanagari visually organises the alphabet based perfectly on the phonetic place of articulation. The dental and retroflex nasals sit as the final "nasal anchor" of their respective five-letter consonant rows (वर्ग / varga).

The Dental Nasal Row (त-वर्ग / ta-varga)
These sounds are spoken with the tongue touching the teeth.
• त (ta), थ (tha), द (da), ध (dha), न (na)
The Retroflex Nasal Row (ट-वर्ग / ṭa-varga)
These sounds are spoken with the tongue curled back against the roof of the mouth.
• ट (ṭa), ठ (ṭha), ड (ḍa), ढ (ḍha), ण (ṇa)

Contains:
Exhaustive notes on change of Sanskrit letter न् to ण् in words
Complete List of Parasmaipada Roots with लोट् i/1 आनि to आणि
List of इदित् Roots with 7.1.58 नुम् न् to ण्
List of Roots with initial ण् to न्