O livro marcado está na estante.

In European Portuguese, definite articles are used more often than in English. Key points:

• O is the masculine singular definite article (“the”).
• Portuguese requires it before most nouns in specific or generic contexts (e.g. O café “coffee” in general, O livro “the book”).
• Omitting the article (saying just “Livro marcado está na estante”) sounds unnatural in Portuguese.

Marcado is the past participle of the verb marcar used here as an adjective.

• As an adjective, it describes the state of the book (“marked”).
• It agrees in gender/number with livro (masculine singular), so we use marcado (not marcada or marcados).
• It comes after the noun because most Portuguese adjectives follow the noun they modify.

Portuguese distinguishes ser and estar by type of information:

• Ser expresses permanent or defining characteristics (“A água é líquida”).
• Estar indicates states or locations.
  Here, está marks the location of the book.
• O livro está na estante = “The book is (located) on the shelf.”
  If you said O livro é na estante, it would be ungrammatical.

Na = em (in/on/at) + a (the, feminine singular).

• Em can translate as “in,” “on,” or “at” depending on context.
• Since estante is feminine singular (a estante), we contract em + a → na.
• Similarly: em + o → no, em + as → nas, em + os → nos.

Adjectives must match the noun’s gender (masculine/feminine) and number (singular/plural).
Examples with marcado:

• Masculine singular: o livro marcado
• Feminine singular: a revista marcada
• Masculine plural: os livros marcados
• Feminine plural: as revistas marcadas

In this sentence, marcado is an adjective describing a state (“the book is marked”).

• Passive voice in Portuguese uses ser
  • past participle (e.g. O livro foi marcado “The book was marked”).
• Estar
  • participle often conveys a resulting state rather than an action in progress.

No, that would sound unnatural.

• Portuguese normally requires the article before singular countable nouns.
• Adjectives almost always follow the noun (“livro marcado”).
• Dropping o or moving marcado before livro (i.e. “marcado livro”) is possible only in poetic or highly stylized contexts, not in everyday speech.

Apply agreement rules and plural forms:

• Masculine plural: Os livros marcados estão na estante.
• Feminine singular (e.g. revista): A revista marcada está na estante.
• Feminine plural: As revistas marcadas estão na estante.

Note that na estante can stay singular if you mean all those marked items are on one shelf. If you had multiple shelves, you’d use nas estantes.