O inverno é a estação em que eu prefiro ficar em casa.

In Portuguese, seasons are often used with the definite article, so o inverno means winter in a general sense.

• O inverno é frio. = Winter is cold.
• Eu gosto do inverno. = I like winter.

English often leaves out the article, but Portuguese commonly keeps it.

Portuguese usually uses an article before nouns in this kind of definition.

So:

• O inverno é a estação... = Winter is the season...

Using a estação makes it sound natural and complete. Leaving out a here would sound wrong in standard Portuguese.

Em que means something like in which or when, depending on the sentence.

In this example:

• a estação em que eu prefiro ficar em casa
• literally: the season in which I prefer to stay at home

Portuguese often uses em que after a noun of time, place, or situation.

Here it connects a estação with the rest of the idea.

Yes, many speakers would also say:

• O inverno é a estação quando eu prefiro ficar em casa.

However, em que is often felt to be more standard or more careful in writing.

A very common alternative is:

• O inverno é a estação em que eu mais prefiro ficar em casa.
• or simply No inverno, eu prefiro ficar em casa.

So quando is understandable and common in speech, but em que is a very solid choice.

Yes, it can be omitted.

Portuguese often drops subject pronouns when the verb form already shows who the subject is.

So both are correct:

• ...em que eu prefiro ficar em casa
• ...em que prefiro ficar em casa

Including eu can add emphasis, clarity, or a slightly more explicit style.

This is very common in Portuguese.

• prefiro = I prefer
• ficar = to stay

After preferir, the next verb usually stays in the infinitive:

• Prefiro ficar em casa. = I prefer to stay home.
• Prefiro sair cedo. = I prefer to leave early.
• Prefiro estudar à noite. = I prefer to study at night.

So prefiro ficar is just prefer to stay.

Ficar em casa means to stay at home or to stay home.

• ficar = to stay, remain
• em casa = at home

This is a very common expression in Portuguese.

Examples:

• Hoje vou ficar em casa. = Today I’m going to stay home.
• Ela prefere ficar em casa no fim de semana. = She prefers to stay home on the weekend.

Em casa is the normal expression for at home.

• Estou em casa. = I’m at home.
• Vou ficar em casa. = I’m going to stay home.

Na casa means in/at the house, usually referring to a specific house, often someone else’s or one being described more literally.

• Estou na casa da minha avó. = I’m at my grandmother’s house.
• Tem muita gente na casa do Pedro. = There are a lot of people at Pedro’s house.

So in your sentence, em casa is the natural choice.

In this sentence, ficar is better.

• ficar em casa = stay home
• estar em casa = be at home

If you say:

• eu prefiro ficar em casa = I prefer to stay home

you are talking about the action or choice of remaining at home.

If you say:

• eu prefiro estar em casa

it means I prefer to be at home, which is also possible, but it focuses more on the state of being at home rather than the decision to stay there.

Both can work, but ficar em casa is especially natural for this kind of preference.

The original sentence is natural, but Portuguese allows other wordings.

For example:

• O inverno é a estação em que eu prefiro ficar em casa.
• No inverno, eu prefiro ficar em casa.
• Eu prefiro ficar em casa no inverno.

These do not all emphasize exactly the same thing:

• The original sentence highlights winter as the season
• No inverno... focuses more directly on during winter
• Eu prefiro ficar em casa no inverno is a simpler everyday version

A rough pronunciation guide for a native English speaker:

• inverno ≈ een-VEHR-noo
• estação ≈ eh-stah-SOWN

A few notes:

• r in inverno is not the strong English r
• ão in estação is nasal, which English does not really have
• the stress is:
  • inVERno
  • estaÇÃO

If you want to sound more natural, the nasal ending in -ão is especially important.

Yes, that is a very close structural translation.

Piece by piece:

• O inverno = Winter
• é a estação = is the season
• em que = in which
• eu prefiro = I prefer
• ficar em casa = to stay at home

So the sentence maps quite neatly to English, even if a more natural English version might be shorter, like Winter is the season when I prefer to stay home.

Yes, it is grammatical and natural, especially in a slightly more careful or written style.

That said, in everyday conversation, Brazilians might more often say something simpler, such as:

• No inverno, eu prefiro ficar em casa.
• Eu prefiro ficar em casa no inverno.

Your original sentence is still completely good Portuguese. It just sounds a bit more structured and descriptive.