A student writes “I was walking home when I saw him” and “I...

Past continuous usually presents a finished sequence, while past simple sets the scene, so was walking signals completion and saw signals background information for the reader.

Past simple and past continuous differ mainly in formality, so the first sentence is informal and the second is formal, with meaning kept stable in narratives.

Past continuous frames an ongoing background, while past simple marks the event within it, which is why was walking pairs naturally with saw in that narrative moment.

Past continuous is preferred for punctual actions, while past simple suits longer processes, so was walking suggests a brief act and walked suggests extended duration.

The two tenses function similarly after when, so both sentences can be treated as equivalent, and variation reflects mainly style and rhythm in narrative writing.