First a quick review of the French imperfect tense (l'imparfait), then a little demonstration of how the tense is used contrastively.
To form the imperfect tense, remove the "-ons" ending from the first-person plural conjugation of a verb to create a stem:
parler: parlons: parl-
choisir: choisissons: choisiss-
vendre: vendons: vend-
prendre: prenons: pren-
appeler: appelons: appel-
lire: lisons: lis-
vouloir: voulons: voul-
savoir: savons: sav-
devoir: devons: dev-
(irregular form) être: ét-
Depending on person and number, add these endings:
In French, the imperfect tense is equivalent to the past progressive tense in English: was ...ing. So:
Je parlais = I was speaking
Tu choisissais = You were choosing
Elle vendait = She was selling
Nous prenions = We were taking
Vous appeliez = You were calling
Ils lisaient = They (masc.) were reading
Je voulais = I was wanting (to)...
Tu savais = You knew (a bit awkward to translate this is as "You were knowing")
Il devait = He had to...
As in English, French verb tenses can be used contrastively. Here's an English example of a contrast between the past progressive and the simple past tense:
I was sleeping when my cell phone rang.
In French, the same contrast is expressed with l'imparfait and le passé composé. To wit:
Je dormais quand mon portable a sonné.
The imperfect tense is used for the "background action," i.e., for actions or events that occur over a period of time. The passé composé, like the simple past tense in English, is used for the "interrupting action," i.e., for actions or events that tend to be sudden and of very short duration. In the above examples, sleeping is the background action; the phone's ringing is the interrupting action.
What if I gave you a problem like this:
Je (regarder) la télé quand le martien (frapper) à la porte.
You'd ask yourself, first, what the background action was: watching TV or the Martian knocking? Obviously, watching TV occurs over a longer period of time than a sudden knock, so regarder should be in the imperfect. Thus:
Je regardais la télé quand le martien a frappé à la porte.
Try this one, which may be a bit more difficult:
Mes copains (arriver) quand je/j' (être) dans la salle à manger.
What's the background action? My being in the dining room or my friends' arriving? It helps to remember that, technically speaking, an arrival happens in a single moment-- the moment the arriving person or thing stops moving. It's only at the very instant that my friends are at the door that I can say they have arrived. Knowing this, we can say that:
Mes copains sont arrivés quand j'étais dans la salle à manger.
My friends arrived when I was in the dining room.
Try your hand at the following sentences.
1. Maman (parler) au téléphone quand notre chat (miauler). (miauler = to meow/mew)
2. Nous (conduire) quand nous (percuter) le cerf. (le cerf = the deer; percuter = to hit, crash into)
3. Quand il (casser) son crayon, je/j' (étudier).
4. Robert et Maxine (skier) quand le bâtiment (exploser). (bâtiment = building)
5. Tu (être) où quand le vol (avoir lieu)? (vol = theft; avoir lieu = to take place)
Final note: The imperfect tense can lead to strange spellings, especially the double-i in the nous form:
Nous étudiions (the imperfect stem of étudier is étudi-)
Nous skiions (the imperfect stem of skier is ski-)