We use Past Continuous
1. for an action that was happening at or around a particular time in the past. It is not important when the action started or when the action finished.
Tom was chatting online yesterday at six o'clock.
2. to emphasise a continuous activity in the past.
It was snowing all day yesterday.
3. for two or more actions that were happening at the same time in the past.
My grandpa was watching TV while my grandma was reading.
4. to give a background information, often at the beginning of the story.
The Moon was shining. George was walking along the road.
5. together with Past Simple
to talk about an action that was in progress when another action happened.
He was playing a computer game when his mum came.
We use Past Continuous for the longer action that was in progress. — He was playing a computer game...
We use Past Simple for the shorter action that happened. — … when his mum came.
We use the word "when" before the action in Past Simple.
Notice when we use a comma (,):
He was playing computer game when his mum came.
When his mum came, hwas playing a computer game.