[Advaita-l] Question on internal Sandhi rule

[Shashi]

Hello everyone, 

I have a question about Sanskrit. It's not really about Advaita per se, 
but there are many learned people here, so I really appreciate the help.

Under what circumstances should a dantavya 'na' (dental) change into a murdhanya 'na' (retroflexal) in a word? For example, why is tRtiya vibhakti of 'Rama' RameNa and not Ramena?

Thank you,

Shashi 

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