@spelling of nasal before stop

For historical reasons, in modern Sanskrit each of these 25 sounds —

Gk ङ्क् Gkh ङ्ख् Gg ङ्ग् Gg ङ्ग् GG ङ्ङ्
Jc ञ्च् Jch ञ्छ् Jj ञ्ज् Jjh ञ्झ् JJ ञ्ञ्
NT ण्ट् NTh ण्ठ् ND ण्ड् Ndh ण्ध् NN ण्ण्
nt न्त् nth न्थ् nd न्द् ndh न्ध् nn न्न्
mp म्प् mph म्फ् mb म्ब् mbh म्भ् mm म्म्

can be spelled in two ways — either starting with a nasal, or starting with an M ं letter.

So the sound nd न्द् can be spelled either as M ंd द् or as n न्d द्. In does not matter how it is spelled, M ं d द् is always pronounced as nd न्द्, even if the M ं and the d द् belong to different words, as in —

tan dadAmi तं ददामि "I'll give it"

The official spelling rule, used in carefully printed books, is —

Spell a J ञ् m म् G ङ् N ण् n न् sound as /M if that sound was made by anusvArasy..., and as /M otherwise.

The practical spelling rule is — unless you are in an university exam, spell every sound nd न्द् as M ंd द्, no one is going to complain and that's more convenient, and clearer in print.

I advise my students to use the practical rule themselves, so that they don't have to waste time figuring out if anusvArasy... was applied or not, and can focus on what they are writing instead. But if they wish to use the official rule I don't complain.

OTOH, I scold them mightily if they dare say ta humdadAmi — even though that pronunciation is PC, it is sort of an archaism.