while vs when

[Sultano]

Since my school days, I have been taught that
while ( past continuous), (past simple)
when( past simple), (past cintinuous)

so, what is the difference between the following sentences:

While I was working in the garden,I hurt my back.
Tom burnt his hand when he was cooking the dinner.

And why don't we say " ..........while he was cooking the dinner."

[Inga]

They arrived while we were having dinner. While she was asleep, thieves broke in and stole her handbag. She met Andy while working on a production of Carmen.
Why not?
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I am an incurable optimist.