Eu prefiro segurar o volante com as duas mãos quando a curva é fechada.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

• Eu prefiro... = I prefer...
• Prefiro... also clearly means I prefer...

Including eu can add emphasis, contrast, or clarity, but it is not required here.

Examples:

• Prefiro dirigir de manhã.
• Eu prefiro dirigir de manhã, mas meu irmão prefere à noite.

After preferir, Portuguese normally uses the infinitive when the next verb has the same subject.

So:

• Eu prefiro segurar... = I prefer to hold...

Not:

• Eu prefiro segurando...

This is similar to English prefer to hold.

More examples:

• Prefiro ficar em casa. = I prefer to stay home.
• Ela prefere estudar à noite. = She prefers to study at night.

Segurar usually means to hold, to grip, or to keep hold of.

In this sentence, it means to hold/grip the steering wheel.

Depending on context, segurar can also mean:

• to hold an object: Segura minha bolsa um instante.
• to restrain/stop: Segura o cachorro!
• to support/bear in some contexts

Here, the meaning is clearly physical grip.

Volante here means steering wheel.

So:

• segurar o volante = to hold the steering wheel

Be careful: volante can have other meanings in other contexts, such as:

• a football/soccer position
• a leaflet/flyer
• something that rotates

But in driving context, o volante is definitely the steering wheel.

Both are possible, but com as duas mãos sounds very natural and specific in Portuguese.

• com as duas mãos = with both hands
• com duas mãos = with two hands

Using as emphasizes that we are talking about the person’s two hands, meaning both hands. It is very common in Portuguese to use the article this way.

Compare:

• Segure com as duas mãos. = Hold it with both hands.
• Ela escreveu com as duas mãos. = She wrote with both hands.

Because mão is an irregular noun.

• singular: a mão
• plural: as mãos

This is just something to memorize. Many learners expect something like mões, but that is incorrect.

Examples:

• Minha mão está doendo.
• Minhas mãos estão frias.

Quando means when.

It introduces a time clause:

• quando a curva é fechada = when the curve is sharp/tight

So the full structure is:

• Eu prefiro segurar o volante com as duas mãos
  +
• quando a curva é fechada

In other words: I prefer to hold the steering wheel with both hands when the turn is sharp.

Yes, literally fechado/fechada often means closed, but in driving/road contexts, curva fechada means a tight or sharp curve/turn.

So:

• uma curva fechada = a tight/sharp turn
• uma curva aberta = a wide/gentle turn

This is a very common contextual meaning. It is one of those cases where the literal translation is not the natural English translation.

Portuguese often uses ser for characteristics, classifications, or inherent descriptions, and estar for temporary states or conditions.

Here, fechada describes the type/nature of the curve:

• A curva é fechada. = The turn is sharp/tight.

That is why é sounds natural.

If you said está fechada, it would usually suggest something more like is closed off, which changes the meaning completely.

Because it agrees with curva, which is a feminine noun.

• a curva = feminine singular
• adjective must match it: fechada

Compare:

• o caminho está fechado = the path is closed
• a estrada está fechada = the road is closed
• a curva é fechada = the curve is tight/sharp

This kind of gender and number agreement is a core part of Portuguese grammar.

Yes, and that may even sound more natural in some situations.

Compare:

• Eu prefiro segurar o volante com as duas mãos quando a curva é fechada.
• Eu prefiro segurar o volante com as duas mãos numa curva fechada.

The second version means something like:

• I prefer to hold the steering wheel with both hands on a sharp turn / in a tight curve.

Your original sentence is correct, but numa curva fechada is a very common alternative.

It can be either, depending on context.

• curva literally = curve
• in driving contexts, English may naturally say turn

So:

• a curva é fechada could be translated as
  • the curve is tight
  • the turn is sharp

Both are valid, depending on how natural you want the English to sound.

A natural approximate pronunciation is:

Eu prefiro segurar o volante com as duas mãos quando a curva é fechada.

Approximate pronunciation:

• eu → eh-o / ew
• prefiro → preh-FEE-roo
• segurar → seh-goo-RAHR
• o volante → oo voh-LAHN-chee
• com as duas mãos → kohng az DOO-as mahngs
• quando → KWAHN-doo
• a curva → a KOOR-va
• é fechada → eh feh-SHA-da

A few pronunciation notes for Brazilian Portuguese:

• d before i often sounds like j in many accents, but there is no such case here.
• te in volante often sounds like chee in many Brazilian accents.
• mãos has a nasal sound; the ão sound is important.
• Stress:
  • preFIro
  • seguRAR
  • voLANte
  • DUas
  • CURva
  • feCHAda