A - Bildung

Das past continuous bildet man mit:
 
were oder was (simple past von "be") +  -ing

B -Gebrauch

Das past continuous bezeichnet eine Handlung, die
gerade in der Vergangenheit stattfindet
(z.B. Erzählung).

Beispiel:	

When he arrived, I was cooking.
What were you doing up so late at night?