Quick comparison

TenseFormUseExample
Past Simple verb + -ed / irregular Short, completed action that interrupted the scene. The phone rang.
Past Continuous was/were + verb + -ing Longer background action already in progress. I was reading a book.
Scene: I was reading (background) when the phone rang (interruption).
She was cooking dinner when he arrived.
They were playing football when it started to rain.

When vs. While

WHEN + Past Simple
WHILE + Past Continuous
I was sleeping when she called.
While I was sleeping, she called.
He fell when he was running.
While he was running, he fell.
Two simultaneous actions: If two long actions were happening at the same time, use while + Past Continuous for both: "She was cooking while I was setting the table." Both were in progress simultaneously.

Exercises

Choose the correct form.

1. I ___ TV when my mum called.

2. She dropped her phone while she ___ down the stairs.

3. ___ they watching a film when you arrived?

4. The lights ___ off while we were having dinner.