Compré veintiocho sellos en correos porque todavía me faltan cartas por enviar.

Because the sentence refers to a completed action in the past: the speaker bought the stamps, and that action is finished.

• compré = I bought
• It is the 1st person singular preterite of comprar

A native speaker is not describing a habit or ongoing background action here, so imperfect would not fit as well.

Compare:

• Compré veintiocho sellos = I bought twenty-eight stamps.
• Compraba sellos = I was buying / I used to buy stamps.

The accent mark shows both stress and distinguishes this form from another one.

• compré = I bought
• compre = present subjunctive form, as in Espero que compre sellos

In compré, the stress falls on the final syllable: com-PRÉ.

In modern Spanish, the numbers from 21 to 29 are normally written as one word:

• veintiuno
• veintidós
• veintitrés
• veintiocho

Historically, these came from forms like veinte y ocho, but standard Spanish now uses the combined spelling.

A couple of them also keep accent marks:

• veintidós
• veintitrés
• veintiséis

But veintiocho does not need one.

Here sellos means postage stamps.

The singular is sello. This word can mean different things depending on context, for example:

• postage stamp
• seal
• stamp/mark
• official stamp

In this sentence, because of cartas por enviar and en correos, the meaning is clearly stamps for mailing letters.

In Spain, Correos is the name of the national postal service, so you will often see:

• en Correos = at the post office / at Correos

With a capital C, it refers clearly to the institution or company.

With lower case, correos, it may be used more loosely to mean the post office / the postal service, but many learners will most often encounter Correos with a capital in Spain.

So:

• en Correos = at the Spanish post office
• lower-case en correos may appear, but the capitalized form is often the clearer standard if you mean the official service

Because en is used here to mean at or in a place.

• en Correos = at the post office
• en casa = at home
• en el banco = at the bank

Using a would usually suggest movement toward a destination:

• Fui a Correos = I went to the post office.

But once you are saying where the buying happened, en is the natural choice:

• Compré sellos en Correos = I bought stamps at the post office.

Proper names often appear without an article, and place/institution names can behave this way too.

So:

• en Correos
• trabajo en Correos

This is similar to how you do not normally say in the London or at the Amazon in English when using a name as a proper noun.

If you use a more generic noun instead, then an article is common:

• en la oficina de correos = at the post office

This is a very common Spanish pattern that works differently from English.

Faltar often means to be lacking / to be missing / to still need.

So:

• me faltan cartas literally means letters are still lacking to me
• natural English meaning: I still have letters left or I still need to send some letters

The important thing is that the thing missing is the subject of the verb:

• Me falta una carta = I still have one letter left / I’m still missing one letter
• Me faltan cartas = I still have letters left / I’m still missing letters

So the verb agrees with cartas, not with me:

• falta with singular
• faltan with plural

Todavía means still.

It shows that the situation continues:

• todavía me faltan cartas por enviar = I still have letters left to send

Without todavía, the sentence would still be grammatical:

• me faltan cartas por enviar

But todavía emphasizes that this is not finished yet.

In many contexts, todavía and aún can both mean still:

• Todavía me faltan cartas
• Aún me faltan cartas

Por + infinitive often means left to do, remaining to be done, or pending.

So:

• cartas por enviar = letters to send / letters left to send

This structure is very common:

• tengo mucho por hacer = I have a lot to do
• quedan platos por lavar = there are dishes left to wash
• hay una tarea por terminar = there is a task left to finish

In your sentence, por enviar describes the letters as not yet sent.

Yes, but the nuance is a little different.

• cartas por enviar emphasizes that the letters are still pending
• cartas que enviar means letters to send, more neutrally

So both are possible, but por enviar often sounds especially natural when talking about something still left undone.

Compare:

• Tengo cartas por enviar = I still have letters left to send
• Tengo cartas que enviar = I have letters to send

Because Spanish often omits the article when talking about things in a general or indefinite sense, especially after certain verbs.

Here cartas means some letters / letters in general, not a specific already-identified set.

So:

• me faltan cartas por enviar = I still have letters to send

If you meant specific letters already known to both speaker and listener, you could say:

• me faltan las cartas por enviar

That would sound more like I still have the letters left to send.

In this sentence, yes, almost certainly.

Carta can mean:

• letter (mail)
• menu
• in some contexts, card

But because the sentence mentions:

• sellos = stamps
• correos = the post office
• por enviar = to send

the meaning is clearly letters in the mail.

Yes, depending on the meaning.

• Fui a Correos = I went to the post office
• Compré sellos en Correos = I bought stamps at the post office
• Compré sellos en la oficina de Correos = I bought stamps at the post office

So:

• a is common with verbs of movement
• en is common when saying where something happened

Both are natural, but they do different jobs.

Yes, it is very normal.

• Compré veintiocho sellos en correos = main clause
• porque todavía me faltan cartas por enviar = reason clause

This is a straightforward structure:

1. action
2. place
3. reason

Spanish could move things around for emphasis, but the given order is neutral and natural.

For example:

• Como todavía me faltan cartas por enviar, compré veintiocho sellos en Correos.

This version puts the reason first, but the meaning stays the same.