O documento está sob o livro.

• O = the (masculine singular definite article)
• documento = document (noun)
• está = is (3rd person singular of estar)
• sob = under / beneath (preposition)
• o = the again
• livro = book (noun)

So the structure is:

article + noun + verb + preposition + article + noun

Because Portuguese normally uses estar for location.

• O documento está sob o livro. = The document is under the book.
• é comes from ser, which is usually used for identity, definition, and more permanent characteristics.

So:

• O documento é importante. = The document is important.
• O documento está sob o livro. = The document is under the book.

Using é here would sound wrong.

It is correct, but it sounds a bit more formal or written than what many Brazilians say in everyday conversation.

In casual spoken Brazilian Portuguese, people often prefer:

• debaixo de
• embaixo de

So in daily speech, you are very likely to hear:

• O documento está debaixo do livro.
• O documento está embaixo do livro.

All of these are understandable, but sob has a more formal or polished feel.

They are very easy to confuse, but they mean almost opposite things in this kind of sentence:

• sob = under / beneath
• sobre = on / on top of / about

Compare:

• O documento está sob o livro. = The document is under the book.
• O documento está sobre o livro. = The document is on the book.

And sobre can also mean about:

• um livro sobre história = a book about history

So this is an important pair to keep separate.

Because both nouns are masculine singular, and here they are being used with the definite article the.

• o documento = the document
• o livro = the book

If the nouns were feminine, you would use a:

• a mesa = the table

If they were plural:

• os documentos = the documents
• os livros = the books

So the article agrees with the noun in gender and number.

Normally, no. That sounds unnatural in standard Portuguese.

Portuguese usually wants an article here:

• O documento está sob o livro.
• Um documento está sob um livro.

English often allows zero article more easily in some contexts, but Portuguese usually does not. For a learner, it is safest to include the article unless you have a specific reason not to.

No. sob does not contract with the article.

So you say:

• sob o livro
• sob a mesa

not something like a merged form.

This is different from:

• de + o = do
• em + o = no
• a + o = ao
• por + o = pelo

That is one reason why the more conversational versions look different:

• debaixo do livro = debaixo de + o livro
• embaixo do livro = embaixo de + o livro

There the contraction happens because of de + o.

Yes. In Brazilian Portuguese, those are often more natural in everyday speech.

• O documento está debaixo do livro.
• O documento está embaixo do livro.

Both are common. In many situations they are basically interchangeable.

A rough guide:

• sob = correct, but somewhat more formal/written
• debaixo de / embaixo de = very common in spoken language

So if your goal is natural conversational Brazilian Portuguese, these alternatives are worth learning.

The accent mark is important.

• está = is (from estar)
• esta = this (feminine)

Examples:

• Ela está aqui. = She is here.
• esta mesa = this table

The accent in está helps mark the stressed syllable and also distinguishes it from another word with a different meaning.

So in your sentence, it must be está, not esta.

A simple learner-friendly approximation is:

oo doh-koo-MEN-too es-TAH sob oo LEE-vroo

A few useful notes:

• documento is stressed on men: do-cu-MEN-to
• está is stressed on the last syllable: es-TÁ
• livro is stressed on the first syllable: LI-vro
• In many Brazilian accents, final o sounds like u, so livro can sound a bit like livru
• The article o often sounds like u in connected speech

So o livro may sound roughly like u livru.

Yes, but the original order is the normal, neutral one.

• O documento está sob o livro.

This is the most straightforward way to say it.

You might sometimes see a different order for emphasis or style, such as:

• Sob o livro está o documento.

That is grammatically possible, but it sounds more marked, literary, or dramatic. For everyday use, stick with the original word order.

It can absolutely be abstract too, and in some abstract expressions it is very common.

Examples:

• sob pressão = under pressure
• sob controle = under control
• sob investigação = under investigation
• sob suspeita = under suspicion

So sob is not limited to literal physical under. In fact, some abstract uses sound especially natural with sob.

documento is correct, but it suggests an actual document, often something official or important.

If you simply mean a sheet of paper lying under a book, Brazilians might also say:

• o papel = the paper
• a folha = the sheet

For example:

• O papel está debaixo do livro.
• A folha está embaixo do livro.

So documento is fine, but the most natural noun can depend on what the object really is.