Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

Background events 

348b

The past progressive may refer to past-time events occurring as a background to

other events which interrupt them. 

The events which occur in the foreground are usually in the past simple:

background

foreground

She was here once, and I 

was baking a cake. And she said, ‘Can I help you?’

Two builders 

were working on our house when it was being built, and a ghost

passed them on the stairs, and they refused to come back and finish the

building. It 

terrified them.

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347d The past simple on sequences of events

Reasons and contexts for events 

348c

The past progressive is often used to give a reason or context for an event:

Her husband 

was doing a PhD at the university, and that’s how I came to meet

him.

A: I rang you yesterday but there was no answer.

B: I was gardening.