IMPERFECT PAST TENSE - VERBS ENDING IN -AR, -ER, IR : (PART 2 ):

B. The imperfect past tense is also used to express an action interrupted in the past . For example:

Eu jantava quando você chegou. I was having dinner when you arrived.
Ela estudava quando ele entrou . She was studying when he came in.

The following construction , however, is more commonly used in conversation:

Eu estava jantando quando você chegou. I was having dinner when you arrived.
Ela estava estudando quando ele entrou. She was studying when he came in.

Ref. Learning Portuguese . Ed. Disal.