Learn Hardcore French: Elle est arrivée à l’aéroport trop tôt, mais son passeport était prêt. - She arrived at the airport too early, but her passport was ready.

In the passé composé, many intransitive verbs of motion or change of state take être as the auxiliary. Arriver is one of them, so you say elle est arrivée. You cannot say elle a arrivé. Other common verbs that take être include: aller, venir, entrer, sortir, partir, rester, naître, mourir, tomber, devenir, revenir, retourner, monter, descendre, passer (when intransitive).

Why does the past participle have an extra -e in arrivée?

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

elle est arrivée (feminine singular, -e)
il est arrivé (masculine singular, no ending)
elles sont arrivées (feminine plural, -es)
ils sont arrivés (masculine plural, -s)

Why is it était prêt (imperfect) instead of a été prêt (passé composé)?

French uses the imperfect (imparfait) for ongoing states or background descriptions in the past, which fits a state like “was ready.” The passé composé focuses on a completed event. So:

son passeport était prêt = his/her passport was (already) in a state of readiness.
son passeport a été prêt = it became ready or was ready at a specific moment, which is a different nuance.

Why is prêt masculine and not prête here?

Adjectives agree with the noun they describe. Passeport is masculine singular, so you use prêt. If the noun were feminine singular (e.g., valise), you’d use prête; if plural, prêts / prêtes.

Why is it son passeport and not sa passeport, even though the owner is female?

French possessive adjectives agree with the gender and number of the thing owned, not the owner. Passeport is masculine singular, so it’s son passeport regardless of whether the owner is male or female. Examples:

son passeport (masc. sing.)
sa valise (fem. sing. starting with a consonant)
son amie (fem. sing. starting with a vowel sound; you still use son to avoid a vowel clash)
ses documents (plural)

Why à l’aéroport and not au aéroport?

Because of elision. Aéroport starts with a vowel sound, so le aéroport becomes l’aéroport. With the preposition à, you get:

à + le = au (before a consonant)
à + la = à la (feminine, before a consonant)
à + l’ = à l’ (before a vowel sound, any gender)
à + les = aux (plural) Hence, à l’aéroport is correct.

Could I say dans l’aéroport instead of à l’aéroport?

Normally you say arriver à l’aéroport (arrive at the airport). Dans l’aéroport emphasizes being inside the building and would be used if you’re talking about something happening inside: e.g., elle est déjà dans l’aéroport (she is already inside the airport). For arrival, à is standard.

Is the placement of trop tôt correct? Could I move it?

Yes, elle est arrivée à l’aéroport trop tôt is very natural. Other natural options:

elle est arrivée trop tôt à l’aéroport (also fine) Less natural/usually avoided: elle est trop tôt arrivée (adverbs like tôt usually don’t go between the auxiliary and the participle).

What’s the difference between trop tôt, très tôt, and en avance?

trop tôt = too early (excessively early, a problem)
très tôt = very early (neutral intensity, not necessarily a problem)
en avance = early/ahead of schedule (neutral/positive; the opposite of en retard)

Can I replace mais with other contrast words like pourtant or cependant?

mais is a coordinating conjunction (but) and is the simplest/most common.
pourtant and cependant mean however; they’re adverbial connectors and typically start the second clause: … Trop tôt; pourtant/cependant, son passeport était prêt.
alors que means whereas/while and introduces a subordinate clause; it contrasts two simultaneous facts, so it’s not the same structure.

Do I need the comma before mais?

It’s standard in French to put a comma before mais when it joins two clauses: …, mais …. Your sentence’s comma is correct.

Any tricky pronunciation points in this sentence?

est arrivée: there’s an obligatory liaison; pronounce the -t: [es-t-arrivée].
trop: final -p is silent here: [tro].
tôt: final -t is silent: [to].
prêt: final -t is silent unless there’s liaison (e.g., prêt à → [prè-ta]); here it’s just [prè].
à l’aéroport: elision makes l’aéroport flow as one; aéro- sounds like [é-ro] at the start.
passeport: double s gives an [s] sound, not [z].

Could I use the literary past (passé simple) instead of the passé composé?

In formal literary writing you might see elle arriva, but in everyday speech and most writing, the passé composé (elle est arrivée) is used.

Is there any risk of confusing prêt (ready) with prêt/preter/prêter?

Yes—note the distinctions:

prêt (adjective) = ready.
un prêt (noun) = a loan.
prêter (verb) = to lend. Context and grammar make the meaning clear; in your sentence, prêt is an adjective agreeing with passeport.

Does arriver ever mean something other than “to arrive”?

Yes. In impersonal constructions, il arrive (que) can mean “it happens (that),” and with an indirect object il m’est arrivé (quelque chose) means “something happened to me.” That’s a different use from physical arrival, but it still uses être in the passé composé (e.g., il m’est arrivé un problème).