When does ㅂ become "m"?

When does ㅂ become "m" sound as in:

감사​합니다 (Gamsaha_M_nida)

Ordinarily ㅂ is “b" or “p” sound.  Is there a module that explains all of these exceptions?

Hi KenA68, 

Our apologies for the late reply over the holidays! You can find a comprehensive list of Korean assimilation rules in lesson 2.9 - 

https://app.rocketlanguages.com/members/products/68/lesson/4534

I hope that helps!