Real stories use both past tenses together. Think of a film:

• the imparfait sets the scene — what was going on, what things were like;
• the passé composé moves the plot — something happens, and it's done.

Je lisais quand le téléphone a sonné. — I was reading when the phone rang.

Imparfait — the scene	Passé composé — what happened
Je dormais…	…quand Bidu a aboyé.
Il faisait beau…	…alors nous sommes allés au parc.
Elle avait vingt ans…	…quand elle est arrivée à Paris.

To stress that something was in the middle of happening, French can also use être en train de + infinitive: J'étais en train de lire quand le téléphone a sonné. — I was in the middle of reading when the phone rang.

New words: sonner — to ring; aboyer — to bark.