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

• the imperfetto sets the scene — what was going on, what things were like;
• the passato prossimo moves the plot — something happens, once, and it's done.

Ero a casa quando ha suonato il telefono. — I was at home when the phone rang.

Imperfetto — the scene	Passato prossimo — what happened
Dormivo…	…quando Bidu ha abbaiato.
Faceva bel tempo…	…così siamo andati al mare.
Marina aveva vent'anni…	…quando è arrivata a Firenze.

For something in progress, use stare in the imperfetto + the gerund (-ando / -endo): Stavo dormendo quando Bidu ha abbaiato. — I was sleeping when Bidu barked. Here dormivo and stavo dormendo mean the same.

New words: suonare — to ring; abbaiare — to bark.