The penultimate vowel of a aṅga (stem) ending in a nasal is lengthened before the affix kvi and before an affix having an indicatory k or ṅ which begins with a consonant other than a semi-vowel or a nasal.,
Thus प्रशान् and प्रतान् from the roots शम् and तम् by 8.2.64. So also before an affix beginning with a झल् consonant (any consonant but a nasal and a semivowel). As शान्तः (with क्त), शान्तवान् (with क्तवतु), शान्त्वा (with क्त्वा), शान्तिः (with क्तिन्). These are all formed with कित् affixes. As to ङित् affixes we have शंशान्तः and तन्तान्तः formed by तस् 3rd Person Dual added to the Intensive roots शम् and तम् ॥ तस् is ङित् by 1.2.4. Why do we say 'ending in a Nasal'? Observe ओदनपक्, पक्वः, पक्ववान् ॥ Why do we say 'before क्वि and jhaladi affix'? Observe गम्यते, रम्यते ॥ Why do we say ङित् or a कित् affix? Observe गन्ता and रन्ता ॥,