Ответ:

  1. When did you go on a picnic? — Last Sunday.
  2. Why did your dad sleep all the way there? — Because he was tired.
  3. When did you eat your lunch? — After our swim.
  4. When did you leave? — At six o’clock.

Вопросы о прошлых действиях образуются с помощью did и начальной формы смыслового глагола.