I was wondering why @Robin asked such an obvious question, until I realized that
to be is irregular in the imperfect tense. The English imperfect has the form of 'was/were + present participle', so
cantaban =
they were singing.
You would thus expect
estaban =
were being , but the
being is omitted. I'll stick my neck out and say to be is the only verb that does that.
(This refers to
to be as the
main verb, not an auxiliary in the passive voice as in
'the guests were being asked to leave')
Crystal clear?