No hables en voz alta en la biblioteca.

Because this is a negative command addressed to tú (informal singular you).

In Spanish, negative commands use the present subjunctive, not the normal present tense.

• hablas = you speak / you are speaking
• no hables = don’t speak

So:

• Hablas en voz alta. = You speak loudly.
• No hables en voz alta. = Don’t speak loudly.

It is a negative informal singular command.

That means:

• negative = it tells someone not to do something
• informal = it is for tú
• singular = it is said to one person

So this sentence is something you would say to one person you would normally address as tú.

That is simply how Spanish grammar works: all negative commands use subjunctive forms.

A useful shortcut is:

• affirmative tú command: usually based on the él/ella/usted present form
  
  • Habla = Speak
• negative tú command: use the present subjunctive
  
  • No hables = Don’t speak

More examples:

• Come = Eat

• No comas = Don’t eat

• Escribe = Write
• No escribas = Don’t write

So no hables follows a very common pattern.

You form it from the present subjunctive.

Steps:

1. Start with the yo form of the present tense:
   hablo
2. Remove the -o:
   habl-
3. Add the subjunctive ending for tú with -ar verbs:
   -es
4. Result:
   hables

So:

• hablar → hablo → habl- → hables

That gives:

• No hables = Don’t speak

Literally, it means in a high voice.

But in natural English, it usually means:

• out loud
• aloud
• in a loud voice
• loudly

It is a very common expression in Spanish.

Examples:

• Lee en voz alta. = Read out loud.
• No lo digas en voz alta. = Don’t say it out loud.

So in your sentence, en voz alta describes how the person is speaking.

Sometimes you may hear hablar alto, and native speakers do use it in some contexts, but en voz alta is a very standard and clear expression.

A useful distinction is:

• hablar alto = to speak loudly
• hablar en voz alta = to speak out loud / aloud / in a loud voice

In a sentence like this one, especially for learners, No hables en voz alta is an excellent, natural choice.

Because the two en phrases do different jobs:

1. en voz alta = tells you how someone is speaking
   → in a loud voice / out loud

2. en la biblioteca = tells you where the action happens
   → in the library

So the sentence structure is basically:

• No hables = don’t speak
• en voz alta = loudly / out loud
• en la biblioteca = in the library

Using en twice is completely normal here.

Spanish often uses the definite article with places and nouns where English might not focus on it as much.

So:

• en la biblioteca = in the library

In Spanish, this sounds complete and natural. Leaving out the article here would usually sound wrong.

You will see the same with many places:

• en la escuela = in the school
• en el banco = in the bank
• en la oficina = in the office

The sentence is addressed to one person using tú.

That means it is:

• singular
• informal

If you wanted other versions, they would be:

• No hables en voz alta en la biblioteca. = to one person, informal
• No hable en voz alta en la biblioteca. = to one person, formal (usted)
• No habléis en voz alta en la biblioteca. = to more than one person, informal in Spain (vosotros)
• No hablen en voz alta en la biblioteca. = to more than one person, formal, or plural in Latin America (ustedes)

Since the prompt is Spanish from Spain, No habléis... is especially useful for the informal plural.

Yes, Spanish allows some flexibility, but the original order is the most neutral and natural.

Standard version:

• No hables en voz alta en la biblioteca.

You could also hear:

• En la biblioteca, no hables en voz alta.

That version puts extra focus on en la biblioteca, almost like:

• In the library, don’t speak loudly.

So the original sentence is the most straightforward version, while other orders may add emphasis.

It depends on tone and context.

Grammatically, No hables is a direct command, just like Don’t speak or Don’t talk in English. It can sound:

• neutral
• firm
• rude

depending on how you say it.

If you want to sound softer, Spanish often adds polite expressions, for example:

• No hables en voz alta, por favor. = Please don’t speak loudly.
• Por favor, no hables en voz alta.

So the sentence itself is not automatically rude, but tone matters a lot.

Yes. If this were written on a sign, Spanish often prefers a more impersonal structure.

For example:

• No se puede hablar en voz alta en la biblioteca. = You can’t speak loudly in the library.
• Se ruega no hablar en voz alta en la biblioteca. = Please do not speak loudly in the library.
• Silencio, por favor. = Silence, please.

By contrast, No hables en voz alta en la biblioteca sounds like you are speaking directly to a specific person.