В аэропорту обмен валюты уже закрылся, поэтому я расплатился картой.

Because аэропорт has a special location form in Russian: в аэропорту.

For some masculine nouns, when they mean a physical location after в or на, Russian uses an old locative ending -у / -ю instead of the regular prepositional ending. Compare:

• в аэропорту = in/at the airport
• в лесу = in the forest
• на мосту = on the bridge

But when you mean about the airport, you use the regular prepositional:

• об аэропорте = about the airport

So here в аэропорту is the normal form for location.

Literally, it is in the airport, but in natural English this is often translated as at the airport.

Russian в is used because the speaker is located inside that place or within that setting. English often prefers at for places like airports, stations, schools, and so on.

So:

• В аэропорту обмен валюты уже закрылся
  = The currency exchange at the airport had already closed
  or
  = The currency exchange in the airport had already closed

Both are fine, but at the airport usually sounds more natural in English.

Here обмен валюты means the currency exchange as a place or service: a booth, counter, desk, or office where you exchange money.

It does not mainly mean the abstract idea of exchanging currency. In real-life Russian, обмен валюты is a very common label for a currency exchange point.

You may also hear the colloquial word:

• обменник = exchange booth / exchange office

So in this sentence, обмен валюты is basically the exchange booth/office.

Because обмен normally takes the genitive: exchange of what? → валюты.

So grammatically:

• обмен
  • genitive

Also, валюта in the singular often means currency in a general sense, not just one single unit. So обмен валюты is the standard set phrase for currency exchange.

You may sometimes see обмен валют, especially in more technical or business-like contexts, but обмен валюты is the usual everyday expression for the exchange service/booth.

Because the grammatical subject is обмен, and обмен is masculine singular.

Even though валюты is next to it, that word is just dependent on обмен. The head noun is обмен.

So the verb agrees with обмен:

• обмен закрылся = masculine singular

That is why you get:

• обмен валюты уже закрылся

not a plural or feminine form.

Because закрыться means to close in the sense to become closed.

Compare:

• закрыть = to close something
• закрыться = to close, to shut down, to become closed

So:

• Я закрыл дверь = I closed the door
• Дверь закрылась = The door closed
• Обмен валюты закрылся = The exchange office closed

In this sentence, the exchange office is not acting on another object; it is itself becoming closed, so закрылся is the right form.

Russian does not have a separate present perfect form like English has closed.

Instead, Russian usually uses:

• past tense
• often with a perfective verb
• sometimes with a word like уже = already

So:

• уже закрылся

can correspond to English:

• already closed
• has already closed
• had already closed

The exact English tense depends on context. In this sentence, English often prefers had already closed, because that earlier action explains the later one:

• The exchange had already closed, so I paid by card.

Поэтому means therefore, so, or that’s why.

It introduces the result of the first clause:

• В аэропорту обмен валюты уже закрылся = The exchange had already closed
• поэтому я расплатился картой = so I paid by card

The comma is standard in Russian here because two clauses are being connected:

• cause/result structure
• first clause + поэтому
  • second clause

So the punctuation is completely normal.

Расплатиться often means to pay up, to settle the bill, or to complete payment, especially in a shop, restaurant, taxi, hotel, and similar situations.

So:

• я расплатился картой = I paid by card / I settled up using a card

Заплатить is also possible in many contexts, but it is a bit broader:

• заплатить за билет = pay for a ticket
• заплатить много денег = pay a lot of money

With картой, расплатиться sounds especially natural for making a payment at the point of sale.

So the difference is roughly:

• заплатить = to pay
• расплатиться = to settle up / pay and finish the transaction

Because картой is instrumental case, and after расплатиться the thing you pay with is put in the instrumental.

The pattern is:

• расплатиться чем? = to pay with what?

Examples:

• расплатиться картой = pay by card
• расплатиться наличными = pay in cash
• расплатиться телефоном = pay with a phone

Карту would be accusative, which would suggest a direct object, but that is not how this verb works here.

So картой marks the means or instrument of payment.

In this sentence it clearly means by card / using a card as the payment method.

That is a very common use of the instrumental case in Russian: it shows the means by which something is done.

So:

• расплатился картой = paid by card

If you wanted to talk about simply having a card with you, Russian would use a different structure, such as:

• с картой = with a card

But here the meaning is definitely payment method, not physical possession.

Because Russian past tense verbs agree in gender.

So:

• я расплатился = the speaker is male
• я расплатилась = the speaker is female

This only affects the first-person past verb. It does not mean anything special beyond grammatical gender.

Notice that закрылся is also masculine, but there it agrees with обмен, not with the speaker.

Yes. Russian word order is flexible, and changing it changes emphasis more than basic meaning.

The given version:

• В аэропорту обмен валюты уже закрылся, поэтому я расплатился картой.

is very natural because it starts with the setting: at the airport.

Other orders are possible, for example:

• Обмен валюты в аэропорту уже закрылся, поэтому я расплатился картой.

This means basically the same thing, but the focus feels slightly different.

So the original order is natural, idiomatic, and probably the most neutral in context.