Questions & Answers about 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.
- 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.