1) We were watching TV when dad came home.
2) Mike was eating dinner when his friend called.
3) You were talking to your friend when I met you in town.
4) She was sleeping when the doorbell rang.
5) The children were playing football when they broke the window
Past Simple - просто глагол во второй форме (если предложение утвердительное)
Past Continuous - глагол to be в одной из форм (am, is, are) и смысловой глагол с окончанием ing (тоже для утвердительного предложения)