Learn Hardcore French: Nous sommes partis en retard parce que Paul a oublié son passeport. - We left late because Paul forgot his passport.

In compound past tenses, most verbs use avoir, but a small, fixed set of (mostly) movement/change-of-state verbs use être. Partir is one of them.
So the correct passé composé is Nous sommes partis.
Other common être verbs: aller, venir, arriver, partir, entrer, sortir, monter, descendre, naître, mourir, rester, retourner, tomber, passer (when intransitive), plus all pronominal verbs (e.g., se lever).

Why does partis have an -s?

With être as the auxiliary, the past participle agrees with the subject in gender and number.

Mixed group or at least one man: Nous sommes partis.
All women: Nous sommes parties.
One man: Je suis parti.
One woman: Je suis partie.

Why doesn’t oublié agree with Paul? Should it be oubliés?

With avoir as the auxiliary, the past participle normally does not agree with the subject. It only agrees with a direct object if that object comes before the verb.

Here the direct object son passeport comes after a oublié, so you keep oublié invariable.
If the object comes before, you agree:
- Paul l’a oublié (l’ = passeport, masculine singular) → oublié
- Paul l’a oubliée (l’ = carte, feminine singular) → oubliée
- Paul les a oubliés/oubliées (plural, masculine/feminine)

Should it be avait oublié (plus-que-parfait) since forgetting happened before leaving?

Both are possible; they give slightly different feels.

Nous sommes partis en retard parce que Paul a oublié son passeport. Neutral, common in conversation: two completed past events, one causing the other.
Nous sommes partis en retard parce que Paul avait oublié son passeport. Highlights that the forgetting happened prior to the departure, often used in more careful narration.

What’s the difference between en retard and tard?

en retard = late relative to a schedule/expectation.
- Je suis en retard. I’m late (for an appointment).
- Nous sommes partis en retard. We left later than planned.
tard = late in the day/clock time.
- Il est tard. It’s late.
- Nous sommes partis tard. We left late (e.g., in the evening).
Related: à l’heure = on time; en avance = early (ahead of schedule).
Transport: Le train est en retard / a du retard. For people arriving late, prefer être en retard.

Could I use car, puisque, or comme instead of parce que?

Yes, but each has a nuance.

parce que: neutral because, works anywhere.
car: rather formal/written, often to justify what precedes. Nous sommes partis en retard, car Paul avait oublié son passeport.
puisque: since/as (reason is assumed known/obvious). Puisque Paul avait oublié son passeport, nous sommes partis en retard.
comme: as/since, must come at the beginning. Comme Paul avait oublié son passeport, nous sommes partis en retard.

Can I put the cause first?

Yes: Parce que Paul a oublié son passeport, nous sommes partis en retard. Put a comma after the first clause.

Why is it son passeport and not sa passeport?

French possessives agree with the gender and number of the thing possessed, not with the owner.

passeport is masculine singular, so son passeport.
If the noun were feminine: sa carte d’embarquement.
Plural: ses documents.

Why is it a oublié (no accent) and not à oublié?

a (no accent) is the 3rd-person singular of the verb avoir: Paul a oublié.
à (with accent) is a preposition meaning to/at and cannot be an auxiliary. Tip: If you can replace a with avait or aura, you need the verb, not the preposition.

How do I make this negative?

Place ne … pas around the auxiliary.

Nous ne sommes pas partis en retard.
Paul n’a pas oublié son passeport. In everyday speech, ne is often dropped: On est pas partis en retard, Paul a pas oublié son passeport (informal). To avoid ambiguity about the cause: Ce n’est pas parce que Paul a oublié son passeport que nous sommes partis en retard.

Any pronunciation tips for this sentence?

Liaison: nous sommes is usually pronounced with a linking z-sound: roughly [nu-zɔm].
partis ends with a silent -s: [paʁti].
Liaison between partis and en: [paʁti-zɑ̃].
retard ends with a silent -d: [ʁ(ə)taʁ].
parce que is commonly pronounced like [paʁskə].
No liaison between sommes and partis (the next word starts with p, a consonant).

Can I say à cause de instead of parce que?

Yes, but the structure and tone change.

With a noun: Nous sommes partis en retard à cause de Paul. This tends to assign blame and sounds negative.
With a clause you’d use parce que, not à cause que: … en retard parce que Paul a oublié son passeport.
Positive cause uses grâce à: Nous sommes arrivés à l’heure grâce au taxi.

Is using on more natural than nous here?

In everyday speech, on is more common for we.

On est partis en retard parce que Paul a oublié son passeport. Agreement note: Many speakers write the past participle in the plural when on clearly means more than one (On est partis/parties). Some very formal styles keep masculine singular (On est parti). In normal modern usage, agreeing (partis/parties) is widely accepted.

When do I use oublier vs oublier de?

oublier + noun/pronoun: forgetting a thing.
- Paul a oublié son passeport.
- Paul l’a oublié.
oublier de + infinitive: forgetting to do something.
- Paul a oublié de prendre son passeport.

What’s the difference between partir, aller, and quitter?

partir: to leave/depart (no direct object). Nous sommes partis à 8 h.
aller: to go (to a place). Nous sommes allés à l’aéroport.
quitter: to leave someone/something (takes a direct object). Nous avons quitté la maison en retard. Because it’s transitive, it uses avoir.

Where does en retard go in the sentence?

After the verb phrase is the most natural spot: Nous sommes partis en retard. Do not say Nous sommes en retard partis. You can front it for emphasis in writing, but it sounds marked: En retard, nous sommes partis (stylistic).

Could I use the imparfait instead of the passé composé?

Use imparfait for ongoing/background or habitual past; passé composé for single, completed events.

Habitual: Nous partions souvent en retard parce que Paul oubliait souvent son passeport.
One occasion: Ce matin-là, nous sommes partis en retard parce que Paul a oublié son passeport.