Nous étudiions. (We were studying.)
Nous skiions. (We were skiing.)
Another quirk to watch out for has to do with the letters c and g. I believe I've explained this in another post, but to reiterate:
When placed in front of the vowels a and o, the consonants c and g undergo a slight change if they're to be pronounced softly (i.e., the "s" sound for c, the "zh" sound for g): the c gains a cedilla, and the g takes on an extra e after it.
Examples: le français, nous mangeons
This is relevant when forming the imperfect tense. Take commencer, for example:
je commençais (note the cedilla)
nous commencions (note the lack of cedilla, because the c is followed by an i, not a or o)
ils commençaient (cedilla again!)
Now watch what happens with manger:
je mangeais (note the additional e)
nous mangions (no e!)
elles mangeaient (e again!)
Keep these changes in mind as you master l'imparfait!