• I was having a shower when the telephone rang. (Je prenais une douche quand le téléphone a sonné)

Le past continuous décrit le contexte (having a shower) alors que le simple past décrit les évènements qui se sont produits (the telephone rang).

Regardons quelques autres exemples:

• I was singing (context) when a neighbor threw water on my head (event).
• People were talking (context) when he entered the room. (event)
• He was entering the room (event) when the fire alarm started to ring. (context)

Here are a few exercises :

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