The imperfect is a pretty simple tense to learn, but is not so easy to use. It is used much less in English than in Italian, hence its relative difficulty for Italian language students. The main point to remember about the imperfect is that there is no indication of the beginning or end of the action, whether the action described is finished or not. That’s why the tense is called “imperfect” or “incomplete”.

Other links that might be of interest
About.com: Italian language
Wikipedia (this is actually a pretty rudimentary reference. I’ve included it to give readers an alternative online source, but don’t think it is terribly thorough)
ielanguages.com

An general outline of this post:
- Common uses
- Regular conjugations
- Regular conjugations (-iare verbs)
- Regular conjugations (reflexives)
- Regular conjugations (esserci)
- Irregular conjugations
- Adverb constructions
- Imperfect continuous

Uses

The imperfect corresponds to the English expressions formed by:
used to + the infinitive / He used to live in London
was/were + the gerund / They were living in Italy
would + the infinitive / She would go to the shops every week

In general, whereas il passato prossimo describes a ‘complete’ outcome, l’imperfetto describes ‘incomplete’ outcomes. The imperfect gives no indication of the beginning or the end of the action or whether it was finished or not; it allows you to refer to incomplete, habitual or recurring actions in the past (”I used to…”, “He was always…” etc):

Something that used to happen
Lavoravo a Roma / I used to work in Rome
Quando abitavo in Italia, andavo a sciare / When I lived (used to live) in Italy, I often went (used to go) skiing
Ci voleva sempre un paio di ore in macchina per arrivare alla stazione di sci piu’ vicina / It always took (used to take) a couple of hours by car to get to the nearest ski resort
Studiava sempre con la sua amica / She used to always study with her friend

Something that was ongoing at a certain moment in the past; in this sense, it is often used after mentre (while)
Mentre leggevo, lui dormiva / While I was reading, he was sleeping
Leggevo il giornale / I was reading the newspaper
Guardavo la televisione / I was watching television

A scene or situation in the past
Faceva caldo e la gente prendeva il sole / It was hot and people were sunbathing

An action that started in the past (and was still going on when something else happened
Lo conoscevo da due anni quando ci siamo sposati / I had known him for two years when we got married to each other

With verbs of thinking, believing and feeling, the imperfect is gneerally used rather than other past forms - the past definite (passato remoto) or present perfect (passato prossimo).
Volevo scrivere / I wanted to write
Mi sentivo male / I felt bad

Regular conjugations

To form the imperfect indicative (imperfetto dell’indicativo) of regular verbs:

(1.) Drop the –re from the infinitive
parlare / to speak…parla-
scrivere / to write…scriv-
finire / to finish…fini-

(2.) Add the following endings to the stem
io / -vo
tu / -vi
lui/lei/Lei / -va
noi / -vamo
voi / -vate
loro / -vano

(3.) Add here are the results:

parlare/ to speak
parlavo  I was speaking, I used to speak
parlavi  you (familiar) were speaking, you used to speak
parlava  he/she/you (formal) was/were speaking, he/she/you used to speak
parlavamo we were speaking, we used to speak
parlavate you (plural) were speaking, you used to speak
parlavano they were speaking, they used to speak

scrivere/ to write
scrivevo I was writing, I used to write
scrivevi you (familiar) were writing, you used to write
scriveva he/she/you (formal) was/were writing, he/she/you used to write
scrivevamo we were writing, we used to write
scrivevate you (plural) were writing, you used to write
scrivevano they were writing, they used to write

finire/ to finish
finivo I was finishing, I used to finish
finivi you (familiar) were finishing, you used to finish
finiva he/she/you (formal) was/were finishing, he/she/you used to finish
finivamo we were finishing, we used to finish
finivate you (plural) were finishing, you used to finish
finivano they were finishing, they used to finish

Regular conjugations (verbs ending in -iare)

For verbs whose infinitives end in –iare (cominciare, mangiare etc), retain the ‘i’ when forming the stem.

cominciare/ to begin - comincia-
cominciavo I was beginning, I used to begin
cominciavi you (familiar) were beginning, you used to begin
cominciava he/she/you (formal) was/were beginning, he/she/you used to begin
cominciavamo we were beginning, we used to begin
cominciavate you (plural) were beginning, you used to begin
cominciavano they were beginning, they used to begin

mangiare/ to eat - mangia-
mangiavo I was eating, I used to eat
mangiavi you (familiar) were eating, you used to eat
mangiava he/she/you (formal) was/were eating, he/she/you used to eat
mangiavamo we were eating, we used to eat
mangiavate you (plural) were eating, you used to eat
mangiavano they were eating, they used to eat

Regular conjugations (reflexive verbs)

Reflexive verbs are conjugated with the usual reflexive pronoun.

lavarsi/ to wash oneself - lava-
mi lavavo I was washing myself, I used to wash myself
ti lavavi you (familiar) were washing yourself, you used to wash yourself
si lavava he/she/you (formal) was/were washing himself/herself/yourself, he/she/you used to wash himself/herself/yourself
ci lavavemo we were washing ourselves, we used to wash ourselves
vi lavavate you (plural) were washing yourselves, you used to wash yourselves
si lavavano they were washing themselves, they used to wash themselves

Regular conjugations (esserci)

The verb form esserci (to be there) has only two forms in the imperfect which are used to acknowledge that something or someone was somewhere.

Singular
C’era Talia? / Was Talia here/there?

Plural
C’erano Talia e Brooke? / Were Talia and Brooke there?

Irregular conjugations

Thankfully, there are not too many irregular verbs to remember; in fact, the only common verb that is completely irregular is essere. I’ve listed a couple of the most common.

bere / to drink
Mental note: if you change the infinitive of bere to bevere, then the conjugation is regular
bevevo I was drinking, I used to drink
bevevi you (familiar) were drinking, you used to drink
beveva he/she/you (formal) was/were drinking, he/she/you used to drink
bevevamo we were drinking, we used to drink
bevevate you (plural) were drinking, you used to drink
bevevano they were drinking, they used to drink

dare / to give
davo I was giving, I used to give
davi you (familiar) were giving, you used to give
dava he/she/you (formal) was/were giving, he/she/you used to give
davamo we were giving, we used to give
davate you (plural) were giving, you used to give
davano they were giving, they used to give

dire / to tell, say
Mental note: if you change the infinitive of dire to dicere, then the conjugation is regular
dicevo I was saying, I used to say
dicevi you (familiar) were saying, you used to say
diceva he/she/you (formal) was/were saying, he/she/you used to say
dicevamo we were saying, we used to say
dicevate you (plural) were saying, you used to say
dicevano they were saying, they used to say

essere / to be
ero I was, I used to be
eri you (familiar) were, you used to be
era he/she/you (formal) was/were, he/she/you used to be
eravamo we were, we used to be
eravate you (plural) were, you used to be
erano they were, they used to be

stare/ to stay, be
stavo I was staying, I used to stay
stavi you (familiar) were staying, you used to stay
stava he/she/you (formal) was/were staying, he/she/you used to stay
stavamo we were staying, we used to stay
stavate you (plural) were staying, you used to stay
stavano they were staying, they used to stay

fare/ to do, make
Mental note: if you change the infinitive of fare to facere, then the conjugation is regular
facevo I was making, I used to make
facevi you (familiar) were making, you used to make
faceva he/she/you (formal) was/were making, he/she/you used to make
facevamo we were making, we used to make
facevate you (plural) were making, you used to make
facevano they were making, they used to make

Here are a couple of examples of irregular conjugations, with somewhat uncommon verb endings.  Worth knowing, regardless.

dedurre/ to deduce
deducevo I was deducing, I used to deduce
deducevi you (familiar) were deducing, you used to deduce
deduceva he/she/you (formal) was/were deducing, he/she/you used to deduce
deducevamo we were deducing, we used to deduce
deducevate you (plural) were deducing, you used to deduce
deducevano they were deducing, they used to deduce

trarre/ to draw, pull  attrarre/ to attract
traevo I was drawing, I used to draw
traevi you (familiar) were drawing, you used to draw
traeva he/she/you (formal) was/were drawing, he/she/you used to draw
traevamo we were drawing, we used to draw
traevate you (plural) were drawing, you used to draw
traevano they were drawing, they used to draw

porre/ to pose  supporre/ to suppose
ponevo I was posing, I used to pose
ponevi you (familiar) were posing, you used to pose
poneva he/she/you (formal) was/were posing, he/she/you used to pose
ponevamo we were posing, we used to pose
ponevate you (plural) were posing, you used to pose
ponevano they were posing, they used to pose

Adverbs

I’ve listed some adverb expressions that indicate continuity and are often associated with the use of the imperfect.
continuamente /continuously
costantemente /constantly
di frequente /frequently
di solito /usually
di tante in tanto /from time to time
la domenica (il lunedi, ecc.) /on Sundays (on Mondays, etc.)
mentre /while
ogni giorno /every day
ogni giorno, ogni notte /each day, each night
ogni tanto /once in a while
qualche volta /sometime
quando /when
sempre /always
spesso /often
tutti i giorni /every day

Imperfect continuous

The imperfect continuous (”I was sleeping”) can be expressed by the imperfect tense of stare, plus the gerund of the main verb.
Stavo dormando quando sei telefonato / I was asleep when you telephoned
But this could be for another post!