Use the past continuous for an action in progress and the past simple for an event during it. When and while help connect the events.
Practical examples
I was cooking when the phone rang.
كنت أطبخ عندما رن الهاتف.
While we were waiting, we checked the map.
بينما كنا ننتظر، راجعنا الخريطة.
She was walking home when it started to rain.
كانت تمشي إلى المنزل عندما بدأ المطر.