Study materials
Here is one short scene. The two tenses in it do completely different work.
I was cooking when the phone rang.
The cooking is background: it was running before the call and after it. The call is an event: a single moment cutting into that background. All past narration is built on these two layers.
Events go in the Past Simple
Everything that moves the story forward stands in the Past Simple. Each such verb is a new step, and the reader moves along with them.
She opened the door, walked in and turned on the light.
The order of the words matches the order of the events in time. That is exactly what makes the Past Simple so convenient for narration.
The background sits in the Past Continuous
Not everything in a story moves. Some things are simply running around it: rain falling, music playing, people talking. That layer takes the Past Continuous.
At eight, I was cooking dinner.
It was raining and the streets were getting dark.
When the action started or finished does not matter here. What matters is that it was already running at the moment you are describing.
When one thing interrupts another
The commonest scene in any story: something was running, and then something happened. The long one goes in the Continuous, the short one in the Simple.
I was cooking when the phone rang.
They were talking when the fire alarm went off.
Mistakes here nearly always come from putting both actions into the same tense. The scene then falls apart: the reader can no longer see what was background and what was the turn.
The mistakes everyone makes
The first is putting both actions in the Past Simple and changing the meaning by accident.
I cooked when the phone rang. → I was cooking when the phone rang.
The second is describing the background with events.
It rained and I walked home slowly. → It was raining and I was walking home slowly.
The third is taking the Continuous for an instant action.
The phone was ringing and I answered. → The phone rang and I answered.
All three are caught by one question. Was this action running, or did it happen? Running is the Continuous; happened is the Simple.
Try it right now
Think of a time when something interrupted you in the middle of a task.
Tell it in English in two sentences: describe what was going on around you in the first, and name the event that changed it in the second.