누구 means “Who” in English, and it reads [noo-goo].
언제 is “When” and reads [uhn-jeh].
어디 is “Where” and reads [uh-dee].
If you combine 누구 and ~이에요? together, then it becomes 누구(이)에요? [noo-goo-eh-yoh?] or 누구입니까? [noo-goo-eem-nee-ggah?] (입니까? is the question version of ~입니다 – see details in previous post.) Then it means, “Who is it?” or “Who is this?” or “Who are you?” To show more respect, you can say “누구세요?” [noo-goo-seh-yoh?]
In the same manner, 언제에요? [uhn-jeh-eh-yoh?] or 언제입니까? [uhn-jeh-eem-nee-ggah?] means “When is it?”
어디에요? [uh-dee-eh-yoh?] or 어디입니까? [uh-dee-eem-nee-ggah?] is “Where is it?” or “Where are you?”
Who is that person? – That person 누구에요? [That person noo-goo-eh-yoh?]
When is the meeting? – The meeting 언제에요? [The meeting uhn-jeh-eh-yoh?]
Where is the conference room? – The conference room 어디에요? [The conference room uh-dee-eh-yoh?]