Ответ:

  1. Somebody new came to our school today.
  2. I haven’t told anybody about our secrets.
  3. Somebody must come and help me with this problem.
  4. Nobody can live without a friend: we all need one.
  5. Has anybody been in the house while we were out?
  6. Nobody answered the teacher’s question.
  7. The house looked empty. There was nobody in it.

Перевод: кто-то, кто-нибудь/никто — выбор определяется утвердительными, вопросительными и отрицательными предложениями.