"That is a dog."
It is a subject particle. 은 after a consonant and 는 after a vowel. There is also the 이/가 pair, which has basically the same role but is used when one wants to emphasize on the subject or when the subject of the sentence is a new information.
I don't understand why this is incorrect. I doesn't say,..that is a dog over there. So why do we use 저.
It is the same endinf we have been seeinf 은(는). The emphasis/topic focus is "that" so THAT is a dog.
Why is the pronunciation of 저갓은 different than what it should be? Anyone feel that?