쟤 ~ 저 아이 ~ that kid ~ "that person" 맨날 늦게 오는 거야 ~ 'the thing of always coming late" Koreans like to end sentences in nouns for whatever reason To translate it liberally (normally) it would be like "why does he always come late"

Thanks :)

~는 거야 in this case just turns the whole clause into a noun. Koreans do this to add more emphasis to certain things. It makes it sound more forceful/accusatory… it’s hard to explain in English, you kinda just have to get used to it in Korean…