Past Simple vs. Past Continuous

1.
When she came home, he cooked dinner.(First she came home, and then he cooked dinner.)
When she came home, he was cooking dinner. (He started cooking dinner and when she came home, he was in the middle of doing that.)

When both actions are in the past simple tense, they are consecutive. That is why past simple is the tense of stories.
When one of the two actions is in the past continuous, it is already in progress at the time the other one takes place.
Example:
She was coming out of the house when the post arrived. (The postman saw her coming out of the house.)
She came out of the house when the post arrived. (First the post arrived, and then she went out.)

2.
She was writing an email last night.   (At some point she was doing that.)
She wrote an email last night.             (She finished it)

An action in the past simple is completed. In this case the email was written entirely.
We do not know whether the action described with the continuous tense was completed or not.
Example:
He was eating the chocolate this morning. (I saw him at it.)
He ate the chocolate this morning. (He ate it all.)

Exercises