How do you form the French imparfait?

Take the nous-form of the present tense, drop the -ons ending, and add: -ais, -ais, -ait, -ions, -iez, -aient. Parler -> nous parlons -> parl- -> je parlais, tu parlais, il parlait, nous parlions, vous parliez, ils parlaient. Finir -> nous finissons -> finiss- -> je finissais. Faire -> nous faisons -> fais- -> je faisais. The whole language has one irregular stem: etre uses et- (j'etais, tu etais, il etait, nous etions, vous etiez, ils etaient).

When do you use the imparfait in French?

Three core uses. First, habitual past actions - 'used to' or 'would' (quand j'etais petit, je jouais au foot = when I was little, I used to play football). Second, ongoing past actions - 'was X-ing' (pendant que je lisais, le telephone a sonne = while I was reading, the phone rang). Third, background description and states - setting the scene, weather, age, time, feelings (il faisait beau, j'avais vingt ans, elle etait fatiguee). Time markers like souvent, toujours, tous les jours, quand j'etais petit all signal imparfait.

French Imparfait

The imparfait is the cleanest tense to conjugate in French. One stem, one set of endings, and exactly one irregular verb in the whole language.

How to form it

Step 1: take the nous-form of the present tense.

Step 2: drop the -ons ending. You're left with the stem.

Step 3: add the imparfait endings.

Person	Ending
je	-ais
tu	-ais
il / elle / on	-ait
nous	-ions
vous	-iez
ils / elles	-aient

Worked through with parler:

nous parlons -> stem parl-
je parlais, tu parlais, il parlait, nous parlions, vous parliez, ils parlaient

With finir:

nous finissons -> stem finiss-
je finissais, tu finissais, il finissait, nous finissions, vous finissiez, ils finissaient

With faire:

nous faisons -> stem fais-
je faisais, tu faisais, il faisait, nous faisions, vous faisiez, ils faisaient

With prendre:

nous prenons -> stem pren-
je prenais, tu prenais, il prenait, nous prenions, vous preniez, ils prenaient

The only irregular

Etre is the only verb in the entire language with an irregular imparfait stem. Use et- with the standard endings.

Subject	Form
je	j'etais
tu	tu etais
il / elle	etait
nous	etions
vous	etiez
ils / elles	etaient

That's it. Every other verb - including avoir, faire, aller, devoir, pouvoir, vouloir - is regular. (Avoir comes out as avais, avait, avions, aviez, avaient because nous avons -> av-; faire is faisais because nous faisons -> fais-. These follow the rule, they aren't exceptions.)

When to use it

Three core uses.

1. Habitual past actions ("used to", "would"):

Quand j'etais petit, je jouais au foot. (When I was little, I used to play football.)
Tous les dimanches, nous mangions chez ma grand-mere. (Every Sunday, we used to eat at my grandmother's.)
Avant, j'habitais a Paris. (I used to live in Paris.)

2. Ongoing past actions ("was X-ing"):

Pendant que je lisais, le telephone a sonne. (While I was reading, the phone rang.)
Quand tu m'as appele, je dormais. (When you called me, I was sleeping.)

3. Background description and states (the scene, weather, age, time, feelings):

Il faisait beau. (The weather was nice.)
J'avais vingt ans. (I was twenty years old.)
Elle etait fatiguee. (She was tired.)
C'etait dimanche matin. (It was Sunday morning.)
La maison etait grande et il y avait un jardin. (The house was big and there was a garden.)

Time markers that trigger the imparfait

If any of these appear in the sentence, imparfait is almost certainly the right tense.

souvent (often)
toujours (always)
tous les jours / toutes les semaines / tous les ans (every day / week / year)
chaque jour / chaque semaine (each day / week)
d'habitude / normalement (usually / normally)
pendant que (while)
quand j'etais petit / jeune (when I was little / young)
a cette epoque (at that time)

The je / il / ils ambiguity

The je, tu, il and ils forms can all sound similar in spoken French (parlais, parlais, parlait, parlaient all reduce to the same /ɛ/ sound). In writing they're distinct; in speech, context and the subject pronoun do the disambiguating.

Worked examples

Quand j'etais a l'ecole, j'aimais les maths. (When I was at school, I liked maths.)
Mon pere travaillait dans une banque. (My father worked at a bank.)
Nous parlions francais a la maison. (We spoke French at home.)
Il faisait froid et il pleuvait. (It was cold and it was raining.)
Tous les etes, nous allions a la plage. (Every summer, we used to go to the beach.)
Elle avait l'air heureuse. (She looked happy.)

Common mistakes English speakers make

Trying to invent irregular forms because the verb is irregular in the present: avoir is regular in the imparfait (j'avais), faire is regular (je faisais), prendre is regular (je prenais). The only irregular stem is et- from etre. Forgetting the -i- in the nous and vous forms - nous parlons in the imparfait is nous parlions, with both an -i- and the -ons (the -i- comes from the imparfait ending, the -ons is part of -ions). And confusing imparfait with passe compose for bounded events - hier j'allais au cinema is wrong, the bounded "yesterday" wants passe compose: hier je suis alle au cinema.