How do you know which consonant to use?
When sounding out words in Korean, how do you know which consonant to use? Or are you not supposed to sound them out like in English? For example, how do I know if it's 잘몰라 or 찰몰라? (for 'I don't know') Because I know that 잘몰라 is correct, but are you just supposed to know the difference between the two words? How do you know when to use ㅁ vs ㅂ? This question probably seems confusing, but I don't know how to word it.

Question

Answer

ㅈ is often used as j and ㅊ as ch. 
For the P, B, and M part..

생일축하합니다. ( saengil chukha habnida)

Written as b but pronounced as m. Its silent.

Answer

I think the giveaway for 잘 몰라 is that 잘 is a word in itself.
I think there's a certain amount of just learning how some words come with Korean.