1) "한국말 할줄아는 사람 모르는대..." = (Someone) said that they didn't know you are a person who can speak Korean.
한국말 할 줄 알다 = from ~(으)ㄹ 줄 알다 = to know how to do something..
한국말 할 줄 아는 사람 = ~는 is added because 알다 is a verb, and because of the ㄹ irregularity, it is dropped when ㄴ is added to it. ~는 is added to allow the verb to modify the noun. So it changes from just a person (사람), to a person that knows how to speak Korean.
모르는대 = ~대 is a colloquial form that is shortened from ~다고 하다 which is used to indicate that someone said something. Since 모르다 is a verb, ~는다고 하다 is shortened to ~는대.
2) 맞긴 한데 = (But) It's right/correct.. (Maybe you said that it will be wrong, but actually in fact it's correct)
This one is 맞다 + ~기는/긴 하다 + 는데.
~긴 하다 is a grammar principle that can be used to downplay what you said. Kind of like to sound a bit modest.
~는데 can used at the end of sentences to express that in actual fact it is the opposite of what was said (so you are a bit surprised).
3) 뒤에 뭐가 오냐에 따라 달라.
This one is 뒤에 뭐가 오다 + ~냐에 따라 다르다. It is the same as ~는지에 따라 다르다.
~는지 = used to express uncertainty, ~에 따라 = according to, 다르다 = to be different. So.. <clause>는지에 따라 다르다 = to be different as according to <the clause>, which can be translated to "it depends on <the clause>.