Why is ㅂ sometimes pronounced as ㅁ? What are the rules?

Question

I'm brand new to learning Korean -- I have studied a little bit of Hangul to learn the sounds, and I just started Lesson 1 of Elementary Korean, 2nd ed. by King & Yeon.

In this lesson there is a list of basic expressions. I am confused in a couple of places where I see the letter ㅂ but the pronunciation guide clearly shows the pronunciation as "m". But in other places, the pronunciation of the same character is "p" or "b" as I would expect.

For example, in Lesson 1, basic expression 5:

...입니다. (English: I'm...)

the pronunciation is shown as "...imnida"

I looked in Sounds of Korean (Choo & O'Grady), but didn't find any rules for when ㅂ should be pronounced as "m" -- maybe I just missed it?

Note: I am trying to avoid Romanization; I only consulted it here when I noticed the discrepancy between what I was hearing on the audio and what I was seeing in Hangul.

Answer 1

This is an example of nasalization: when ㅂ or ㅍ as the final consonant of a syllable is followed by an initial ㄴ or ㅁ in the next syllable, the ㅂ or ㅍ is pronounced as ㅁ.

See this table for additional rules that show how pronunciation can change. See also section 4.9 and 4.9.1 in Sounds of Korean for more information on nasalization.

Answer 2

As I studied about phonetic when ㅂ or ㅍ meet ㄴ or ㅁ pronunciation chang to ㅁ like this word 앞문 but the way we pronunce is 암문.