Quiero volver a Lisboa algún día.

In Spanish, subject pronouns are often omitted because the verb ending already shows who the subject is.

• Quiero = I want
• The -o ending tells you it is yo

So Yo quiero volver a Lisboa algún día is also correct, but yo is usually unnecessary unless you want emphasis, contrast, or clarity.

For example:

• Quiero volver a Lisboa algún día. = neutral
• Yo quiero volver a Lisboa algún día. = adds emphasis, like I want to go back someday

The a is needed because volver often uses a before a destination when it means to go back / return to a place.

• volver a Lisboa = to return to Lisbon
• volver a casa = to go back home
• volver al trabajo = to go back to work

Without a, the sentence would sound incorrect here.

Important: this is not the personal a. It is just the preposition to used with a destination.

Here, volver means to return or to go back.

So:

• Quiero volver a Lisboa = I want to go back to Lisbon
• volver on its own often means to return
• volver a + place = to go back to + place

It can also appear in other structures, such as:

• volver a + infinitive = to do something again
  • Volví a leer el libro. = I read the book again.

But in your sentence, volver a Lisboa clearly means returning to a place.

Because Spanish uses its own name for the city: Lisboa.

Many place names change from one language to another. For example:

• London → Londres
• Lisbon → Lisboa
• Seville → Sevilla

So if you are speaking Spanish, Lisboa is the normal form.

Algún día means someday or one day in the sense of at some point in the future.

It expresses an indefinite future time:

• Quiero volver a Lisboa algún día. = I want to go back to Lisbon someday.

It does not usually mean a specific day. It is vague and open-ended.

This is because alguno shortens to algún before a singular masculine noun.

This shortening is called apocope.

• algún día = correct
• alguno día = incorrect

Compare:

• algún libro = some book
• algún problema = some problem
• alguno = used when the noun is not directly after it, or when it stands alone

Since día is a singular masculine noun, Spanish uses algún.

Good question, because it looks feminine, but día is masculine:

• el día
• un día
• algún día

Most nouns ending in -a are feminine, but not all of them. Día is one of the common exceptions.

Other examples of masculine nouns ending in -a include:

• el problema
• el mapa
• el sistema

So in this sentence, algún día is masculine because día is masculine.

Yes. Regresar is also correct and means to return / to go back.

• Quiero volver a Lisboa algún día.
• Quiero regresar a Lisboa algún día.

Both are natural. In many contexts, volver is especially common and very everyday. Regresar is also perfectly normal, though sometimes it can sound a little more formal depending on the region and context.

In Spain, volver is extremely common in everyday speech.

Because city names usually do not take an article in Spanish.

So you normally say:

• a Lisboa
• en Lisboa
• desde Lisboa

Just like in English, you usually do not say the Lisbon.

Some place names do take an article, but most city names do not.

Yes, but the original order is the most neutral and natural:

• Quiero volver a Lisboa algún día.

You could also hear:

• Algún día quiero volver a Lisboa.
• A Lisboa quiero volver algún día.

These alternatives change the emphasis:

• Algún día at the beginning emphasizes someday
• A Lisboa at the beginning emphasizes the destination

The original sentence is the most straightforward way to say it.

A Spain pronunciation would be roughly:

KYE-ro bol-BER a leez-BO-a al-GOON DEE-a

A few useful notes:

• qu in quiero sounds like k
• ie in quiero is a diphthong, something like kye
• v and b are pronounced very similarly in Spanish
• volver has stress on the second syllable: vol-VER
• algún has stress on the last syllable because of the written accent
• día has two syllables: dí-a

In standard Peninsular Spanish, the s in Lisboa is a normal s sound.

Yes, often it can. But it means one day in the sense of someday, not necessarily on one particular day.

So:

• Algún día volveré. = I’ll come back one day / someday.

If you want to refer to a specific single day in a sequence, Spanish would usually express that differently depending on context.

So for your sentence, someday is usually the clearest English match.

Yes, it is completely natural and idiomatic.

Quiero volver a Lisboa algún día sounds like something a native speaker would genuinely say in conversation. It is simple, correct, and very common in tone and structure.

You could use it in many situations, for example when talking about travel, memories, or future plans.