Wah How to use Past Continuous and Past Simple.

We use Past Continuous and Past Simple forms to describe two event in the past which happen at the same time.

We use Past Continuous to indicate that a longer action in the past that was interrupted.
We use Past Simple for a single action that take a short time to interrupt the longer action in the past.

• I was watching a television when the phone rang. 
• While we were sleeping, it started to rain.
• She was cooking dinner when I got home.
• I got to the office while Sara was just making some coffee.
• The computer suddenly went off when I was writing the email.
• I was attending the conference when I met my ex-boyfriend.
• What were you doing when John arrived?
• What were you doing when the lights went off last night?

When and While
We use when before Past Simple.
We use when or while before Past Continuous.

It was snowing when I went home.