Study this example situation:
Yesterday Sarah and Jane played tennis. They began at 9 o’clock and finished at 11. So, at 10 they were playing tennis.
They were playing = “they were in the middle of playing”. They had not finished playing.
Was/were -ing is the past continuous:
We use the past continuous to say that somebody was in the middle of doing something at a certain time. The action or situation had already started before this time but had not finished:
- This time last year I was living in Paris.
- What were you doing at 10 o’clock the day before yesterday?
- I waved to her but she wasn’t looking.