Past Continous.

They were playing tennis yesterday morning.

"They were playing" means that they were in the middle of playing tennis. They had started playing, but they hadn't finished.

This is the past continous tense:

I/she/he was
We/they/you were

Here are some examples of past continous:

1. This time last year I was living in Brazil.

2. What were you doing at 10:00 yesterday?

More examples of Simple Past

The past continous does not tell us whether an action was finished or not. Perhaps it was finished, perhaps not. Compare:

Dan was cooking dinner. (past continous) = He was in the middle of cooking dinner. We don't know whether he finished.

Dan cooked dinner. (simple past) = He began and finished cooking.

We often use the past continous (I was doing) and simple past (I did) together to say that something happened in the middle of something else:

For example:

1. My friend was waiting for me, as I left from school.

2. Virat hurt his back while he was catching the ball.