Here's a quick reminder of the rules of using past simple and past continuous together.
In this sentence there are two verbs:
I was watching television when he arrived.
The first action is a long action - it lasted for a period of time. We therefore use the past continuous.
The second action is a short action that has interrupted the first. So for this we need past simple!
The regular past simple is formed using 'ed' and the past continuous is formed using was/were + past participle.
Now I've reminded you of how to use these tenses, try the exercise below and see if you can decide which verb needs which tense. Good luck!
Lesson by Caroline