Learn Hardcore Spanish (Spain): Aún necesitamos aportar más dinero, porque los billetes de tren son caros. - We still need to contribute more money, because the train tickets are expensive.

aún (with accent) = “still” or “yet.”
aun (without accent) often means “even” (e.g. aun los mejores cometen errores).
In your sentence, you want “still,” so you must write aún with an accent.

Can I replace aún with todavía?

Yes. In Spain aún and todavía are interchangeable when meaning “still” or “yet.”
Example: Todavía necesitamos aportar más dinero is just as correct and natural as Aún necesitamos aportar más dinero.

Why is aún placed before the verb instead of after it?

Spanish adverbs like aún can appear before or after the main verb or verb phrase, but placement can affect emphasis:

Aún necesitamos aportar… (emphasis on “still”)
Necesitamos aportar aún más dinero… (emphasis on “even more”)
In your sentence, putting aún at the very start stresses that you still need more money.

Why does the sentence use necesitamos aportar instead of tenemos que aportar or debemos aportar?

All three express necessity, but with subtle differences:

tener que + infinitivo → external obligation (“we have to”).
deber + infinitivo → moral obligation or strong advice (“we should”).
necesitar + infinitivo → need or necessity (“we need to”).
Here necesitar aportar focuses on the actual requirement (they lack funds), not just an imposed duty.

What exactly does aportar mean in this context? Is it the same as “to pay”?

Aportar = “to contribute” or “to provide” (your share of money).
It isn’t exactly the same as pagar (“to pay”), which implies settling a bill. Aportar suggests adding your portion to a common fund.

Could I use poner más dinero instead of aportar más dinero?

Yes, poner más dinero is a more colloquial way to say “put in more money.”

aportar sounds a bit more formal or precise.
poner is everyday spoken Spanish and works perfectly: Aún necesitamos poner más dinero.

Why is it más dinero and not más de dinero?

Use más directly before an uncountable noun: más agua, más tiempo, más dinero.
You reserve más de + [number or pronoun] for specific quantities or pronouns, e.g. más de 50 euros, más de lo que esperaba.

Why does the sentence say los billetes de tren instead of just los billetes?

Adding de tren specifies which tickets you mean. Without it, los billetes could be bus tickets, plane tickets, even raffle tickets.
los billetes de tren = “train tickets.”

Why is the adjective caros used with ser instead of estar?

Spanish uses ser for inherent or permanent qualities, including price or value:

Este coche es caro. (It’s expensive by nature.)
Estar would imply a temporary state (e.g. está caro might sound like “it’s expensive right now for some reason”).
Here we talk about the general high cost of train tickets, so ser caros is correct.

Is billete the same word used throughout the Spanish-speaking world?

Not always. In Spain, billete is standard for train, bus or plane tickets. In many Latin American countries people say boleto for the same thing. The meaning is identical.