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

• the imperfeito sets the scene — what was going on, what things were like;
• the perfeito moves the plot — something happens, once, and it's over.

Eu estava em casa quando o telefone tocou. — I was at home when the phone rang.

Imperfeito — the scene	Perfeito — what happened
Eu dormia…	…quando o Bidu latiu.
Fazia sol…	…então a gente foi à praia.
Ela tinha vinte anos…	…quando chegou ao Rio.

For something in progress, Brazilians very often use estar in the imperfect + the -ndo form: Eu estava dormindo quando o Bidu latiu. — I was sleeping when Bidu barked. Here dormia and estava dormindo mean the same.

New word: latir — to bark.