deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
According to the teachings of modern professors, the sound nd न्द् must be spelled as nd न्द् whenever /pANini says that it must be compulsorily pronounced nd न्द्, and must be spelled as M ं + d द् whenever pANini says that you may pronounce it either as nd न्द्, or as hum + d द्. This rule is used in carefully printed books.
According to the usage of many speakers, spelling it always as M ं + d द् is peachy. You will find this way of spelling in old books.
According to the usage of university students who took Sanskrit 101, you spell M ं + d द् when the n न् sound comes from a padafinal m म् or from /sam, and nd न्द् elsewhere. This rule-of-thumb works 99.9% of the time like the professors' rule above.