In the room of a yar letter (every consonant except ha final) in a pada , when Nasal follows, there is optionally a Nasal substitute.,
The word पदान्त is understood here. Thus वाक् + नयति = वाग्नयति or वाङ्नयति, अग्निचिद्नयति or अग्निचिन्नयति, त्रिष्टुब्नयति or त्रिष्टुम्ननयति, श्वलिण्नयति or श्वलिड्नयति ॥
Why do we say 'final in a Pada'? Observe, वेद् + मि = वेद्मि ॥ Here there is no option. So also क्षुभ्नाति ॥
Vart:- When it is a pratyaya or affix that follows, the nasalisation is obligatory in the secular language. Thus वाक् + मात्रम् = वाङ्मात्रम्, कियन्मात्रम् ॥ It is, however, only before the affixes मय and मात्र that the change is obligatory, and not before every affix beginning with a nasal.,