Prefiero pasar la tarde en casa para descansar.

Because preferir is a stem-changing verb in the present tense.

The infinitive is preferir (to prefer), but in many present-tense forms the e in the stem changes to ie:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

So prefiero means I prefer.

Pasar la tarde means to spend the afternoon.

Here, pasar does not mean just to pass in the physical sense. In expressions of time, it often means to spend:

• pasar el día = to spend the day
• pasar la noche = to spend the night
• pasar el fin de semana = to spend the weekend

So Prefiero pasar la tarde... means I prefer to spend the afternoon...

In Spanish, when talking about parts of the day or specific time periods in this kind of expression, the definite article is often used.

So Spanish says:

• pasar la tarde = literally to spend the afternoon
• pasar la mañana = to spend the morning
• pasar la noche = to spend the night

English sometimes drops the in similar expressions, but Spanish usually keeps it.

En casa is the normal Spanish expression for at home or in the house/home, especially when speaking generally about being at one’s home.

• Estoy en casa. = I’m at home.
• Prefiero quedarme en casa. = I prefer to stay at home.

If you say en la casa, it usually sounds more like in the house as a specific building, often to distinguish it from somewhere else:

• Los niños están en la casa, no en el jardín. = The children are in the house, not in the garden.

So here, en casa is the natural choice.

Para + infinitive expresses purpose: in order to, so as to, or simply to.

So:

• para descansar = to rest / in order to rest

The whole sentence means that the reason for spending the afternoon at home is rest.

Other examples:

• Estudio para aprender. = I study to learn.
• Voy al supermercado para comprar comida. = I go to the supermarket to buy food.

No, not in this sentence.

Use para + infinitive when you mean in order to do something.

• Prefiero pasar la tarde en casa para descansar. = I prefer to spend the afternoon at home to rest.

Porque means because, and it normally introduces a conjugated verb, not an infinitive:

• Prefiero pasar la tarde en casa porque estoy cansado. = I prefer to spend the afternoon at home because I’m tired.

So:

• para descansar = to rest / in order to rest
• porque estoy cansado = because I’m tired

This is very common in Spanish.

The first verb is conjugated:

• prefiero = I prefer

The second verb stays in the infinitive:

• pasar = to spend

So prefiero pasar means I prefer to spend.

This pattern works with many verbs:

• Quiero comer. = I want to eat.
• Necesito dormir. = I need to sleep.
• Prefiero quedarme. = I prefer to stay.

Yes, absolutely.

That would mean something like I prefer to stay at home to rest.

The difference is slight:

• pasar la tarde en casa focuses on spending the afternoon at home
• quedarme en casa focuses on staying at home

Both are natural, but the original sentence emphasizes how the afternoon will be spent.

No, it is widely understood and used across the Spanish-speaking world.

A learner of Spanish from Spain will certainly hear it, but it is not exclusive to Spain. It is a standard Spanish expression.

Yes, depending on context.

Descansar usually means to rest, especially after effort or tiredness, but in natural English it may sometimes be translated as to relax.

Examples:

• Necesito descansar. = I need to rest.
• Quiero ir a la playa para descansar. = I want to go to the beach to relax/rest.

In your sentence, descansar suggests taking it easy, recovering energy, or relaxing.

Spanish does not use a separate word equivalent to English to before an infinitive after every verb.

In English:

• I prefer to spend

In Spanish:

• Prefiero pasar

After verbs like preferir, querer, necesitar, decidir, Spanish usually just uses the infinitive directly.

Yes, in many contexts prefiero can be translated naturally as I prefer or I’d rather.

For example:

• Prefiero pasar la tarde en casa.
  • I prefer to spend the afternoon at home.
  • I’d rather spend the afternoon at home.

The best English translation depends on tone and context, but the Spanish is the same.

Yes, but that is normal in Spanish.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• (Yo) prefiero = I prefer

You could say Yo prefiero pasar la tarde en casa para descansar, but in most cases yo is unnecessary unless you want emphasis or contrast:

• Yo prefiero pasar la tarde en casa, pero él prefiere salir. = I prefer to spend the afternoon at home, but he prefers to go out.

It breaks down like this:

• Prefiero = I prefer
• pasar la tarde = to spend the afternoon
• en casa = at home
• para descansar = to rest / in order to rest

So the structure is:

[conjugated verb] + [infinitive phrase] + [place] + [purpose]

That makes the sentence very typical and useful as a model for building your own:

• Prefiero estudiar en la biblioteca para concentrarme.
• Prefiero quedarme aquí para ayudarte.
• Prefiero ir temprano para evitar el tráfico.

Yes, but the original order is the most natural.

Most natural:

• Prefiero pasar la tarde en casa para descansar.

You may also hear:

• Para descansar, prefiero pasar la tarde en casa.

That version gives extra emphasis to the purpose: To rest, I prefer to spend the afternoon at home.

In everyday speech, the original sentence is smoother and more neutral.

Put no before the conjugated verb:

• No prefiero pasar la tarde en casa para descansar.

Grammatically that is correct, but in real life Spanish speakers would often say something more natural, such as:

• Prefiero no pasar la tarde en casa. = I prefer not to spend the afternoon at home.
• No quiero pasar la tarde en casa. = I don’t want to spend the afternoon at home.

So the exact negative version depends on what you want to say.

You can simply use question marks and intonation:

• ¿Prefieres pasar la tarde en casa para descansar? = Do you prefer to spend the afternoon at home to rest?

Notice that for you, the verb changes:

• prefieres = you prefer

Spanish does not need an extra word like do to form the question.