Learn Hardcore Portuguese (Portugal): O leitor pergunta sobre o livro.

Leitor is the masculine form: o leitor = the (male) reader or the reader in a generic, grammatically masculine way.
The feminine form is leitora: a leitora = the (female) reader.

In many contexts, o leitor can refer either to a specific male reader or to a generic reader whose gender isn’t specified, because Portuguese often uses the masculine as the default grammatical gender.

Why is it O leitor and not Um leitor? What difference would Um leitor pergunta sobre o livro make?

O leitor pergunta sobre o livro
- O is the definite article (“the”).
- This suggests a specific reader that both speaker and listener can identify (e.g. the reader of this magazine, the reader we were just talking about), or “the reader” as a general role (like in a book review or an article: the reader asks about the book).
Um leitor pergunta sobre o livro
- Um is the indefinite article (“a”).
- This would mean “A reader asks about the book”, introducing a new, non‑specific reader that hasn’t been identified yet.

So the choice between o and um is the usual definite vs. indefinite distinction, very similar to the reader vs a reader in English.

Could this sentence also be A leitora pergunta sobre o livro? What changes?

Yes.

O leitor pergunta sobre o livro = The (male / generic) reader asks about the book.
A leitora pergunta sobre o livro = The (female) reader asks about the book.

Changes:

The article changes: o → a (masculine → feminine).
The noun changes: leitor → leitora (masculine → feminine).
The verb pergunta stays exactly the same, because it only agrees with person and number, not gender:
- ele pergunta / ela pergunta = he asks / she asks.

Why does the sentence use pergunta sobre o livro and not pergunta o livro?

In Portuguese, the verb perguntar normally follows this pattern:

perguntar algo a alguém (to ask something to someone)
perguntar a alguém sobre algo (to ask someone about something)

Here we only see the part about the topic:

perguntar sobre o livro = to ask about the book

You cannot say ✗ perguntar o livro to mean ask about the book; that sounds like you are trying to “ask the book” itself, which is not how the verb works.

So the preposition sobre is necessary to introduce the topic of the question.

What does sobre mean in this sentence, and could I replace it with de or acerca de?

Here, sobre means about / concerning:

perguntar sobre o livro = to ask about the book

Possible alternatives:

perguntar acerca do livro
- acerca de also means about / regarding.
- acerca do = acerca de + o (contracted).
- This is a bit more formal or written style, but correct.
perguntar do livro
- This is not natural here.
- de can sometimes mean “about” in Portuguese, but with perguntar, you normally use sobre or acerca de, or por in certain expressions.

So the most neutral and common options in European Portuguese are:

perguntar sobre o livro (most usual)
perguntar acerca do livro (more formal / literary)

I’ve seen perguntar por as well. What’s the difference between perguntar sobre o livro and perguntar pelo livro?

Both sobre and por can appear with perguntar, but they usually suggest different nuances:

perguntar sobre o livro
- Focus on the content or topic: questions about what the book is like, what happens in it, who wrote it, etc.
- He asked about the book itself.
perguntar pelo livro (from perguntar por + o livro → pelo livro)
- Often focuses on the whereabout / availability / situation of the book:
  - Is it there?
  - Has it arrived?
  - Can I have it?
- Example meaning: He asked after the book / asked if the book was available / asked for the book.

So:

sobre o livro → about its content / subject.
pelo livro → about its presence, state, or obtaining it (though context is key).

Why do we say o livro and not just livro? Could I say O leitor pergunta sobre livro?

Portuguese tends to use the definite article much more often than English, especially with singular countable nouns.

O leitor pergunta sobre o livro
- o livro is the normal, idiomatic form.
✗ O leitor pergunta sobre livro
- This sounds unnatural in standard European Portuguese.
- Omitting the article here would feel incomplete, except in some special, more abstract uses (like in certain fixed expressions or titles).

Even if in English you might say asks about books (plural, generic) or asks about a book, in Portuguese you would typically still use an article:

pergunta sobre livros (plural, generic)
pergunta sobre um livro (a book, one book)
pergunta sobre o livro (the book, a specific book or one previously mentioned)

Could the word order be different, like O leitor sobre o livro pergunta?

No, not in normal, natural Portuguese.

The standard word order for a simple declarative sentence is:

Subject – Verb – Rest of the sentence

So:

O leitor (subject)
pergunta (verb)
sobre o livro (prepositional phrase: “about the book”)

✗ O leitor sobre o livro pergunta is grammatically odd and sounds like poetry gone wrong or a mistake. Prepositional phrases like sobre o livro typically come after the verb in this type of sentence.

You could, however, change the emphasis a bit with intonation or add more information around it, but the basic order O leitor pergunta sobre o livro is the normal one.

How is the verb pergunta being used in terms of tense? Does it mean “asks” or “is asking”?

Pergunta is the 3rd person singular, present indicative of perguntar.

In Portuguese, the present tense often covers both:

He asks (habitual / general present)
He is asking (right now, at this moment)

So O leitor pergunta sobre o livro can mean:

The reader asks about the book.
or
The reader is asking about the book.

Context and sometimes adverbs (e.g. agora, now) show which nuance is intended.

If I want to replace o livro with a pronoun, what is more natural: O leitor pergunta sobre ele or something like O leitor pergunta‑lhe sobre o livro?

Both structures exist, but they do different things:

Replacing “the book” with a pronoun:
- O leitor pergunta sobre ele.
- ele here refers to the book (grammatically masculine: o livro → ele).
- Meaning: The reader asks about it (the book).
Adding an indirect object (the person being asked):
- O leitor pergunta‑lhe sobre o livro.
- lhe = to him / to her in European Portuguese.
- Meaning: The reader asks him/her about the book.

So:

If you want to avoid repeating o livro, you use ele:
- O leitor pergunta sobre ele.
If you want to mention the person being asked, you use a clitic pronoun like lhe in EP:
- O leitor pergunta‑lhe sobre o livro.

In spoken European Portuguese, people may also use stressed forms:

O leitor pergunta a ele / a ela sobre o livro.

Is this sentence the same in Brazilian Portuguese, or is there any difference?

The sentence O leitor pergunta sobre o livro. is perfectly natural in both European Portuguese (Portugal) and Brazilian Portuguese.

Main points:

Vocabulary (leitor, pergunta, sobre, o livro) is shared.
Structure [subject] + [verb] + sobre + [object] is the same.

Differences would appear more in:

Pronoun use:
- European Portuguese: O leitor pergunta‑lhe sobre o livro.
- Brazilian Portuguese often prefers: O leitor pergunta para ele/ela sobre o livro.

But the original sentence itself works unchanged in both varieties.