O outono é a estação em que eu gosto de caminhar mais, porque a cidade fica mais tranquila.

In Portuguese, seasons are often used with the definite article, so o outono means autumn / the autumn.

• o outono = autumn
• a primavera = spring
• o verão = summer
• o inverno = winter

In English, we usually say just autumn or fall, but Portuguese often prefers the article.

Here, estação means season.

That word can also mean station in other contexts, so learners sometimes get confused. The meaning depends on the sentence:

• a estação do ano = the season of the year
• a estação de trem = the train station

In O outono é a estação..., it clearly means season.

Because estação is a feminine noun in Portuguese.

So you use feminine words with it:

• a estação
• a estação tranquila
• esta estação

This is something you just have to learn with the noun. The ending -ão does not always mean a word is masculine.

Em que here means something like in which or when.

So:

• a estação em que eu gosto de caminhar mais
  = the season in which I like to walk more = more naturally in English, the season when I most like to walk

Portuguese often uses em que after nouns of time or place.

You could also hear:

• a estação quando eu gosto mais de caminhar

But em que sounds very natural and standard here.

Yes, it could be omitted.

Portuguese often drops subject pronouns when the verb form already makes the subject clear:

• eu gosto = I like
• gosto = I like

So both are possible:

• a estação em que eu gosto de caminhar mais
• a estação em que gosto de caminhar mais

Including eu can add a little emphasis or simply sound more explicit. In Brazilian Portuguese, subject pronouns are used more often than in European Portuguese, so eu sounds completely normal.

Because the verb gostar normally requires the preposition de before the thing that is liked.

• gostar de algo = to like something
• gostar de fazer algo = to like doing something

Examples:

• Eu gosto de café. = I like coffee.
• Eu gosto de caminhar. = I like walking.

So de is required here.

After gostar de, Portuguese uses the infinitive when talking about liking an activity in general.

• gosto de caminhar = I like to walk / I like walking
• gosto de ler = I like to read / I like reading
• gosto de viajar = I like to travel / I like traveling

This is very similar to English, where both to walk and walking are possible.

Here mais means more.

The idea is that autumn is the season in which the speaker likes walking more than in other seasons.

So the sentence suggests a comparison, even though the second part is not said explicitly:

• gosto de caminhar mais
  = I like walking more
  = I like walking more in autumn than at other times

It does not mean the most by itself. If you wanted to say the season when I like walking the most, you could also say:

• O outono é a estação de que eu mais gosto para caminhar.
• O outono é a estação em que eu mais gosto de caminhar.

Notice that mais can move around depending on what is being emphasized.

Yes, it could.

In this sentence:

• eu gosto de caminhar mais

the mais naturally modifies the idea of liking to walk more in that season.

But Portuguese also allows:

• eu mais gosto de caminhar

That version sounds more like the season when I most like to walk.

So both are possible, but they may sound slightly different in emphasis:

• gosto de caminhar mais = I walk / like walking more
• mais gosto de caminhar = it is the season when I most like to walk

The original sentence is very natural and easy to understand.

Because ficar often means to become, to get, or to end up being.

So:

• a cidade é tranquila = the city is calm
• a cidade fica mais tranquila = the city becomes / gets calmer

Here the idea is that during autumn, the city changes state and becomes calmer than usual.

This is a very common use of ficar in Portuguese:

• ficar feliz = to become happy
• ficar cansado = to get tired
• ficar frio = to get cold

Because tranquila agrees with cidade, and cidade is feminine.

• a cidade → feminine
• tranquila → feminine adjective form

Compare:

• o bairro fica mais tranquilo = the neighborhood gets calmer
• a cidade fica mais tranquila = the city gets calmer

Adjectives in Portuguese often change form to match the gender and number of the noun.

The comma separates the main idea from the reason that follows.

• O outono é a estação em que eu gosto de caminhar mais
• porque a cidade fica mais tranquila

This is similar to English punctuation in many cases, where a clause giving a reason is often separated for clarity.

The comma is very natural here, especially because the first part of the sentence is fairly long.

Yes. Porque means because, but there are other possible connectors depending on style.

For example:

• pois = because / since
• já que = since / given that
• como = since (usually placed earlier in the sentence)

Examples:

• ... porque a cidade fica mais tranquila.
• ... pois a cidade fica mais tranquila.
• Como a cidade fica mais tranquila, o outono é a estação em que eu gosto de caminhar mais.

But porque is the most direct and common choice here.

Usually, no.

In Portuguese, names of seasons are normally written in lowercase:

• outono
• inverno
• primavera
• verão

So O outono is correct, not O Outono, unless it begins a sentence or is part of a title that uses capitalization rules.

Yes, it sounds natural and correct.

A Brazilian speaker might also say slightly different versions, such as:

• O outono é a estação em que eu mais gosto de caminhar, porque a cidade fica mais tranquila.
• No outono, eu gosto mais de caminhar, porque a cidade fica mais tranquila.

These are just stylistic variations. The original sentence is perfectly good Brazilian Portuguese.