Quiero pedir perdón por hablar en voz alta en la biblioteca.

Because querer + infinitive is the standard way to say to want to do something in Spanish.

• Quiero = I want
• pedir = to ask for / to request

So:

• Quiero pedir perdón = I want to ask for forgiveness / I want to apologise

This is very common in Spanish:

• Quiero comer = I want to eat
• Quiero salir = I want to go out

Spanish often uses this structure where English also uses want + to + verb.

Pedir perdón is a fixed expression meaning to apologise, literally to ask for forgiveness.

Breakdown:

• pedir = to ask for, to request
• perdón = forgiveness, pardon

So although the literal meaning is to ask for forgiveness, in normal English the best translation is often simply to apologise.

Other similar expressions:

• Perdón = Sorry / Excuse me / Pardon
• Pedir disculpas = to apologise
• Disculparse = to apologise

In this sentence, pedir perdón sounds natural and sincere.

Here, por means for in the sense of because of or on account of.

• por hablar en voz alta = for speaking loudly / for talking out loud

You are apologising because you spoke loudly, so por is the correct preposition.

Compare:

• Pido perdón por llegar tarde = I apologise for arriving late
• Gracias por venir = Thanks for coming

By contrast, para usually expresses purpose:

• Estudio para aprender = I study in order to learn

So por hablar is about the reason for the apology, not the purpose.

After por, Spanish often uses the infinitive when talking about an action in a general sense.

So:

• por hablar en voz alta = for speaking loudly

Spanish does not normally use the -ing form here the way English does.

Compare:

• Gracias por ayudarme = Thanks for helping me
• Lo castigaron por mentir = He was punished for lying

So even though English says for speaking, Spanish prefers por + infinitive:

• por hablar

En voz alta is the normal expression for out loud, aloud, or in a loud voice.

Breakdown:

• voz = voice
• alta = high / loud
• en voz alta = in a loud voice / out loud

You cannot usually say hablar alto to mean to speak loudly in standard Spanish from Spain in the same way as this sentence. The natural expression is:

• hablar en voz alta = to speak out loud / to speak loudly

A few examples:

• No leas en voz alta = Don’t read out loud
• Habló en voz alta = He/She spoke loudly

So en voz alta is an idiomatic chunk worth learning as a whole.

Because en means in or inside, and here it tells you where the speaking happened.

• en la biblioteca = in the library

Use a when there is movement to a place:

• Voy a la biblioteca = I’m going to the library

Use en when someone is in that place:

• Estoy en la biblioteca = I’m in the library

In your sentence, the person is apologising for speaking loudly in the library, so en is correct.

In Spanish, the definite article is used more often than in English. Here, la biblioteca means the library.

Spanish usually includes the article when referring to a place like this:

• en la biblioteca = in the library
• en el colegio = at school / in the school
• en la oficina = in the office

Leaving out the article would sound incomplete here.

Yes, absolutely. Quiero disculparme por hablar en voz alta en la biblioteca is also correct and natural.

There are slight differences in tone:

• pedir perdón = to ask forgiveness; often sounds more personal or serious
• disculparme = to apologise; very common and neutral
• pedir disculpas = also very common and polite

So these are all possible:

• Quiero pedir perdón por hablar en voz alta en la biblioteca.
• Quiero disculparme por hablar en voz alta en la biblioteca.
• Quiero pedir disculpas por hablar en voz alta en la biblioteca.

They are similar, but pedir perdón can feel a bit stronger or more heartfelt.

Because alta agrees with voz, and voz is a feminine noun.

• la voz = the voice
• voz alta = loud voice

In Spanish, adjectives usually agree in gender and number with the noun they describe:

• libro alto = tall/high book
• mesa alta = tall/high table

So:

• en voz alta uses alta because voz is feminine singular.

Not exactly.

• hablar en voz alta = to speak loudly / out loud
• gritar = to shout / yell

So hablar en voz alta suggests speaking louder than is appropriate, but not necessarily shouting.

In a library, even talking normally may be wrong, but hablar en voz alta especially suggests that the person was being noticeably loud.

Yes. Quiero pedir perdón... literally means I want to apologise / I want to ask forgiveness...

It can sound like:

• an introduction to the apology
• a deliberate, sincere way of beginning
• slightly formal or thoughtful

If you want a more direct apology, you could say:

• Pido perdón por hablar en voz alta en la biblioteca.
• Perdón por hablar en voz alta en la biblioteca.
• Lo siento por hablar en voz alta en la biblioteca.

So Quiero pedir perdón... frames the apology as an intention, while still functioning as a real apology.

It sounds polite, natural, and somewhat sincere. It is not extremely dramatic, but it does sound more thoughtful than a casual perdón.

Tone-wise:

• Perdón = quick, casual apology
• Lo siento = sorry
• Pido disculpas = more formal
• Quiero pedir perdón = sincere, personal, slightly weightier

In Spain, this sentence would fit well if someone wants to apologise properly, especially in a situation like breaking the silence in a library.