A while ago someone asked me in a comment about adding -은/는요 and where in the Sogang books it is introduced. I had a rough idea, but now I have been looking through my books (every single page!) to find the exact point, and now I have an answer… Finally…:
In the grammar booklet to Sogang 1B on page 46 (in the vocabulary section) there is a note (one of the green boxes). -은/는요 is thus not treated as a separate grammar point, it is just introduced through actually using it in a dialogue.
For those who are not familiar with this, it is used to ask the same question back to a person who has asked you about something.
By adding -은/는요 to the person’s name, you can omit actually repeating the question since it’s already implied that you are asking the same question.
For instance if someone asks you what you did during the weekend and you after telling about your weekend ask “what about you?” in English, the other party will know that you are asking about their weekend. That is the function of 은/는요 🙂