yo chistaba
I used to mumble / I was mumbling
tú chistabas
you used to mumble / you were mumbling
él chistaba
he used to mumble / he was mumbling
nosotros chistábamos
we used to mumble / we were mumbling
vosotros chistabais
you used to mumble / you were mumbling
ellos chistaban
they used to mumble / they were mumbling