A Ana senta-se ao lado da Maria na biblioteca.

In European Portuguese, it is very common to use the definite article before a person’s name.

• A Ana = Ana
• a Maria = Maria

This does not usually make it more specific in the way the does in English. It is just a normal feature of Portuguese, especially in everyday speech.

A few notes:

• A is used before feminine names.
• O is used before masculine names.
• In some contexts, especially very formal writing, the article may be omitted.
• This use is much more typical in Portugal than in some other varieties.

So A Ana is completely normal in European Portuguese.

Because the verb here is sentar-se, which means to sit down or to seat oneself.

The -se is a reflexive pronoun. It shows that the subject is doing the action to herself:

• A Ana senta-se = Ana sits down

Without -se, sentar can be used differently, often with the idea of seating someone/something:

• A professora senta os alunos. = The teacher seats the students.

So in this sentence, senta-se is the natural form because Ana herself is sitting down.

This is a very important feature of European Portuguese.

In a simple affirmative sentence, European Portuguese normally puts object and reflexive pronouns after the verb. This is called enclisis.

So:

• A Ana senta-se... is the normal European Portuguese pattern.

English speakers often expect something like se senta, but in Portugal that is usually not the default in a plain affirmative main clause.

Very roughly:

• European Portuguese: senta-se
• Brazilian Portuguese: se senta is much more common

In European Portuguese, se tends to move before the verb only in certain environments, for example after words like:

• não
• já
• quem
• que
• some subordinating conjunctions

Example:

• A Ana não se senta ao lado da Maria.

So senta-se is the expected European Portuguese form here.

Ao lado de is a fixed expression meaning next to or beside.

It is built like this:

• a(o) lado de = at the side of

In practice, you should learn it as one chunk:

• ao lado de alguém = next to someone
• ao lado de alguma coisa = next to something

Examples:

• ao lado da Maria = next to Maria
• ao lado do carro = next to the car

It works much like an expression rather than a word-for-word construction.

Ao is a contraction of:

• a + o = ao

Here, lado is a masculine noun, so it goes with o:

• a o lado → ao lado

This is just standard contraction in Portuguese.

Other common contractions of this type are:

• a + a = à
• de + o = do
• de + a = da
• em + o = no
• em + a = na

So ao lado is the normal contracted form.

Because de combines with the article a that appears before Maria.

Step by step:

• de + a Maria
• becomes da Maria

So:

• ao lado da Maria = next to Maria

This happens because, in European Portuguese, names often take an article, and prepositions usually contract with that article.

Other examples:

• gosto da Ana = I like Ana
• venho da casa da Maria = I come from Maria’s house

So da Maria is simply de + a Maria.

Because na is the contraction of:

• em + a = na

So:

• na biblioteca = in the library

This is very common in Portuguese. Prepositions often contract with articles.

Examples:

• no carro = em + o carro
• na escola = em + a escola
• do livro = de + o livro
• da casa = de + a casa

Since biblioteca is feminine, the article is a, and em + a becomes na.

Yes, senta-se is in the present indicative.

The verb is:

• sentar-se
• ela senta-se = she sits down / she sits

Depending on context, the Portuguese present can express different things, such as:

• a habitual action
• a general fact
• something happening now
• a vivid narrative style

So this sentence could mean something like:

• Ana usually sits next to Maria in the library
• Ana is sitting down next to Maria in the library

The exact interpretation depends on context. Portuguese often uses the simple present more flexibly than English.

Yes.

• sentar-se focuses on the action of sitting down, or the act of taking a seat.
• estar sentado / estar sentada describes the state of already being seated.

Examples:

• A Ana senta-se ao lado da Maria. = Ana sits down / sits next to Maria.
• A Ana está sentada ao lado da Maria. = Ana is seated next to Maria.

In real use, the first sentence can sometimes sound a bit broader and may also be understood as sits in the sense of takes her place there, while está sentada clearly describes position or state.

For European Portuguese, that would sound unnatural in a simple sentence like this.

The standard European Portuguese order is:

• A Ana senta-se ao lado da Maria na biblioteca.

Putting se before the verb is more typical of Brazilian Portuguese, or of European Portuguese only in certain grammatical contexts.

So if you are learning Portuguese from Portugal, the sentence you should model is:

• A Ana senta-se...

not

• A Ana se senta...

The given order is natural, but Portuguese word order is fairly flexible.

This sentence is structured like:

• A Ana = subject
• senta-se = verb
• ao lado da Maria = position relative to Maria
• na biblioteca = place

You could move parts around for emphasis, for example:

• Na biblioteca, a Ana senta-se ao lado da Maria.
• A Ana senta-se na biblioteca ao lado da Maria.

These alternatives are possible, but the original version is very natural and clear.

The main thing to preserve is the reflexive form senta-se and the correct contractions like ao, da, and na.