What is the easiest way to choose between passe compose and imparfait?

Ask whether the action is bounded. If it has a clear start and end in the past (yesterday, last week, in 2010, for two hours, suddenly), use passe compose: hier, j'ai mange une pizza. If it's habitual, ongoing, or background description with no end specified (every day, while, when I was young, used to), use imparfait: je mangeais une pizza tous les dimanches. The same English sentence can map onto either depending on context. 'I ate pizza' could be 'j'ai mange une pizza' (one specific occasion) or 'je mangeais de la pizza' (used to, repeatedly).

Which past tense does French use for descriptions and states?

Imparfait, almost always. Descriptions of the setting (il faisait froid, la maison etait grande), descriptions of people (elle etait jeune et belle), states (j'avais peur, il etait fatigue, je voulais partir), age (j'avais dix ans), time (il etait minuit), weather (il pleuvait) all take imparfait. The exception is when the state has a clear bounded start - 'j'ai eu peur quand le chien a aboye' (I got scared when the dog barked) uses passe compose because 'getting scared' is a moment, not an ongoing state.

Passe Compose vs Imparfait

The two simple French past tenses split English's single past tense between them. The rules are clear once you have them.

The decision rules

Passe compose for:

Completed events in a closed time frame (hier, la semaine derniere, en 2020)
Actions with a specified duration that has ended (j'ai travaille deux heures, j'ai vecu la cinq ans)
A single event, or a sequence of events forming a narrative
The interrupting event in a "while X was happening, Y happened" structure
Sudden changes of state (j'ai eu peur, il s'est mis a pleuvoir)

Imparfait for:

Habitual or repeated actions (every day, every Sunday, used to)
Ongoing past actions ("was X-ing")
Background description (the scene, the weather, ages, time of day)
Mental and emotional states (savoir, vouloir, penser, croire as background states)
The interrupted action in a "while X was happening, Y happened" structure

Time markers as triggers

Triggers passe compose	Triggers imparfait
hier	souvent
hier soir	toujours
la semaine derniere	tous les jours
l'annee derniere	chaque dimanche
en 2020	pendant que
il y a deux ans	quand j'etais petit
une fois	a cette epoque
soudain / tout a coup	d'habitude
ce matin	normalement

If one of these markers appears, the tense choice is mostly decided. When neither appears, you decide by context.

Side-by-side examples

Passe compose	Imparfait	Notes
Hier, j'ai mange une pizza.	Je mangeais de la pizza tous les dimanches.	One occasion vs habit
J'ai habite a Paris cinq ans.	J'habitais a Paris quand j'etais etudiant.	Bounded duration vs background
Je suis alle au cinema hier soir.	J'allais au cinema chaque vendredi.	One event vs habit
Marie est arrivee a huit heures.	Quand je suis arrive, Marie parlait au telephone.	Bounded events vs the ongoing one
Il a fait tres chaud samedi.	Il faisait tres chaud.	Closed past vs setting the scene
J'ai eu une voiture rouge (bought and sold).	J'avais une voiture rouge (the one I owned).	Bounded ownership vs ongoing state

The interruption pattern

A characteristic French past-tense structure: imparfait for the ongoing background action, passe compose for the event that interrupts it.

Quand je suis arrive, Marie dormait. (When I arrived, Marie was sleeping.)
Pendant que je parlais avec ma mere, le telephone a sonne. (While I was talking with my mother, the phone rang.)
Je prenais une douche quand l'eau s'est coupee. (I was taking a shower when the water cut off.)

The imparfait sets the stage; the passe compose delivers the event. This is the bedrock structure of French narration.

States vs events: the verb shift

A small group of common verbs shifts meaning between the two tenses. Worth memorising as a set.

Verb	Imparfait (state)	Passe compose (event)
savoir	je savais = I knew (a fact)	j'ai su = I found out, learnt
connaitre	je connaissais = I knew (someone)	j'ai connu = I met (for the first time)
vouloir	je voulais = I wanted	j'ai voulu = I tried, decided to
pouvoir	je pouvais = I could	j'ai pu = I managed to
avoir	j'avais = I had (state)	j'ai eu = I got, received
devoir	je devais = I was supposed to	j'ai du = I had to (and did)

Examples:

J'ai connu ma femme en 2010. (I met my wife in 2010 - the event.)
Je connaissais Marie depuis l'ecole. (I'd known Marie since school - the ongoing state.)
J'ai su la verite hier. (I found out the truth yesterday - moment of finding out.)
Je savais la verite depuis des mois. (I had known the truth for months - state.)
J'ai voulu partir. (I decided to leave / I tried to leave.)
Je voulais partir. (I wanted to leave - ongoing wish.)

These distinctions don't translate cleanly. French is doing semantic work in the choice of tense.

Worked examples

L'annee derniere, je suis alle en Italie et j'ai visite Rome. (Last year I went to Italy and visited Rome.) - bounded passe compose chain
Quand j'etais petit, ma famille et moi allions a la plage chaque ete. (When I was little, my family and I used to go to the beach every summer.) - habitual imparfait
Marie preparait le diner quand je suis rentre. (Marie was cooking dinner when I came home.) - imparfait + passe compose
Le restaurant etait plein et il faisait tres chaud. (The restaurant was full and it was very hot.) - background imparfait
J'ai commence a etudier a huit heures et j'ai fini a onze heures. (I started studying at eight and finished at eleven.) - bounded passe compose
Quand le telephone a sonne, je dormais profondement. (When the phone rang, I was deeply asleep.) - interruption

Common mistakes English speakers make

Defaulting to passe compose for everything because it feels closer to English simple past. Imparfait covers habitual and ongoing past, and missing it is the surest marker of an English-speaking learner. Conversely, using imparfait for a bounded event because the duration is long: j'ai habite a Paris cinq ans is passe compose (bounded duration, ended), not imparfait. And missing the meaning-shifting verbs - j'ai connu Marie and je connaissais Marie are genuinely different sentences and shouldn't be used interchangeably.