Ответ:

  1. They are shopping at the supermarket. They aren't cooking dinner.
  2. He is doing his homework. He isn't watching TV.
  3. They are playing with the fish. They aren't fishing.
  4. She is texting a friend. She isn't reading.

Для Present Continuous используется форма am/is/are + глагол с окончанием -ing. В отрицании добавляется not.