Das ist + Nominativ. Question: Who/what is that? Das ist ein Haus. Das bin ich. Das bist du. The (real) question is, why in English they say 'That's me.'. But for that you have to ask in the english section ;)

Hello, can you ask your question in English, maybe? I don´t speak Mandarin. :-)

Ah, very good question! I had to think first for a while. "Das" is the subject, but "ich" is a subject-predicativum, connected with a copula verb (https://de.wikipedia.org/wiki/Kopula) and in this construction, the part which is more specific (and that is the "ich") takes the lead and determines the form of the verb. So, how would you build the same sentence, just with "wir" instead of "ich"?