6. On Monday Mary (cooked / was cooking) lunch when the postman (brought/ was bringing) her a letter from her friend.

6. On Monday Mary was cooking lunch when the postman brought her a letter from her friend.

В данном предложении необходимо использовать Past Continuous для действия, которое происходило в определенный момент в прошлом (Mary was cooking lunch), и Past Simple для действия, которое завершилось (the postman brought her a letter).

Ответ: was cooking / brought