In this particular sentence do before and when mean the same and can be used interchangeably?
They were married for 12 years before/when they died/got divorced.
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Yes, they do mean the same thing.
While they can be used interchangeably, the choice of conjunction may be influenced by what follows.
They had been married for 12 years when they died in an accident.
When here refers to a specific point in time.
They had been married for 12 years before they decided to split up and go their own ways.
Before here suggests that the decision was considered over a period of time.
However, these are nuances and reflect the writer's preferences rather than any rule.