Na primavera, o bairro fica mais bonito, e em setembro as árvores voltam a ficar verdes.

Because na is the contraction of em + a.

• em = in
• a primavera = the spring
• em a primavera becomes na primavera

This kind of contraction is very common in Portuguese:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

So na primavera literally comes from in the spring.

Because months usually appear without an article in Portuguese when you are just saying in September, in October, etc.

So:

• em setembro = in September
• em outubro = in October

You would only use an article in more specific contexts, but in a normal sentence like this, em setembro is the natural choice.

Here fica comes from ficar, which often means to become, to get, or to end up being depending on context.

In this sentence:

• o bairro fica mais bonito = the neighborhood becomes / gets prettier

So ficar is not exactly to be in the permanent sense. It suggests a change of state.

Compare:

• O bairro é bonito. = The neighborhood is beautiful.
• O bairro fica mais bonito. = The neighborhood gets more beautiful / becomes prettier.

Because ficar emphasizes that something changes.

• é mais bonito would describe a state or comparison
• fica mais bonito means it becomes prettier

In spring, the neighborhood is not just being described; it is visibly changing, so fica is a very natural choice.

Because bonito is an adjective meaning beautiful or pretty, and its comparative is made with mais:

• bonito = beautiful
• mais bonito = more beautiful / prettier

melhor is the comparative of bom:

• bom = good
• melhor = better

So:

• mais bonito = prettier
• melhor = better

They are not interchangeable.

Yes, bairro usually means neighborhood or district.

It refers to an area within a city or town. In Brazilian Portuguese, bairro is a very common everyday word.

Examples:

• Moro em um bairro tranquilo. = I live in a quiet neighborhood.
• Esse bairro é antigo. = That neighborhood is old.

This is a very common Portuguese structure:

• voltar a + infinitive = to do something again
• ficar verdes = to become green

So:

• as árvores voltam a ficar verdes = the trees become green again

The first verb, voltam, shows the idea of again. The second verb, ficar, expresses the change of state.

This structure is extremely useful:

• voltar a chover = to rain again
• voltar a trabalhar = to work again
• voltar a ser feliz = to be happy again

Yes, you could, and it would sound natural in many contexts.

• voltam a ficar verdes
• ficam verdes de novo

Both can mean become green again.

The difference is mostly one of style and nuance:

• voltar a + infinitive often sounds a little more structured and emphasizes returning to a previous state
• de novo is very common in speech and simply means again

So both work, but voltam a ficar verdes is especially elegant and natural in descriptive writing.

Because verdes agrees with as árvores, which is plural.

In Portuguese, adjectives usually agree in gender and number with the noun they describe.

• a árvore verde = the green tree
• as árvores verdes = the green trees

Since árvores is plural, the adjective must also be plural: verdes.

Because the subject is as árvores, which is third person plural.

The verb voltar must agree with the subject:

• a árvore volta = the tree returns / becomes again
• as árvores voltam = the trees return / become again

So voltam is used because árvores is plural.

Because Na primavera is an introductory time expression.

In Portuguese, it is very common to separate an opening time phrase with a comma, especially in clear, written style:

• Na primavera, o bairro fica mais bonito.
• De manhã, eu estudo.
• No verão, faz muito calor.

The comma helps the sentence flow and makes the time setting clear right away.

Because voltar a + infinitive is the normal structure for saying to do something again.

• voltam a ficar verdes = become green again

If you said voltam verdes, it would not sound natural for this meaning. It would feel incomplete or would suggest something different.

Portuguese usually needs the infinitive here:

• voltar a crescer = to grow again
• voltar a funcionar = to work again
• voltar a ficar verdes = to become green again

In this context, it means to become green.

The verb ficar has several meanings, including:

• to stay
• to become
• to end up

You have to choose the meaning from context.

Here, because the trees were not green and then change back, ficar verdes means become green.

Examples:

• Fiquei feliz. = I became happy.
• Ela ficou em casa. = She stayed at home.

Same verb, different meanings.

Mais is the regular word for more in comparisons.

So:

• bonito = beautiful
• mais bonito = more beautiful / prettier

This is how most Portuguese adjectives form the comparative:

• mais alto = taller
• mais interessante = more interesting
• mais bonito = prettier / more beautiful

Only some adjectives have irregular comparatives, such as:

• bom → melhor
• mau → pior
• grande → maior
• pequeno → menor

Bairro is roughly pronounced like BY-oo in Brazilian Portuguese, though the exact sound depends on accent.

A few helpful notes:

• ai sounds like the y in my
• rr in Brazilian Portuguese is usually a strong h-like sound
• the final o is often pronounced like oo in many Brazilian accents

So bairro does not sound like English barrio. The rr is different, and the word is only two syllables: BAI-rro.