Habla en voz baja.

Here it is most naturally understood as an informal singular command: something you say to one person you address as tú.

So Habla en voz baja. = Speak quietly.

Spanish has a small wrinkle here: habla can also be the present tense form meaning he/she speaks or you speak (formal usted in some contexts, depending on structure). The form is identical, so context tells you which meaning is intended.

In a real situation, if you say Habla en voz baja directly to someone, it is understood as a command.

Because Spanish usually omits subject pronouns when they are not needed.

So instead of saying Tú habla en voz baja, Spanish normally just says:

Habla en voz baja.

The verb form already tells you who the command is for. In fact, adding tú in a command often sounds unnecessary unless you want emphasis, contrast, or emotion.

Because en voz baja is a very common Spanish expression meaning in a low voice or quietly.

Literally:

• voz = voice
• baja = low
• en voz baja = in a low voice

Spanish often uses this structure where English would prefer an adverb like quietly.

You may sometimes hear things like hablar bajo in some contexts, but for learners, hablar en voz baja is the safest and most standard phrase.

Here en is part of the fixed expression en voz baja.

It does not translate word-for-word in a neat way, but the idea is:

• en voz alta = out loud / in a loud voice
• en voz baja = quietly / in a low voice

So en is helping form a phrase of manner: it tells you how the speaking is done.

Because baja describes voz, and voz is a feminine singular noun.

So the adjective has to agree:

• voz = feminine singular
• baja = feminine singular

That is why you get:

• voz baja not
• voz bajo

Baja is an adjective.

It describes the noun voz:

• voz baja = low voice

The whole phrase en voz baja then functions like an adverbial expression, telling you how someone speaks.

This sentence uses the tú command form, so it is for one person informally.

Other versions are:

• Hable en voz baja. → formal singular (usted)
• Hablad en voz baja. → informal plural in Spain (vosotros)
• Hablen en voz baja. → formal plural, or plural in regions that use ustedes for all plural you

Since you asked about Spanish from Spain, hablad is the standard informal plural form there.

A simple guide:

• Habla → AH-bla
  The h is silent.
• voz → roughly both, but with a pure o sound
  In most of Spain, z sounds like the th in thin.
• baja → roughly BA-ha
  The j is the strong Spanish sound, like the ch in Scottish loch or German Bach.

A rough pronunciation guide for Spain would be:

AH-bla en both BA-ha

That is only approximate, but it is a useful starting point.

Yes, absolutely.

Adding por favor makes it sound more polite:

• Habla en voz baja. → Speak quietly.
• Por favor, habla en voz baja. → Please speak quietly.

The plain imperative is not automatically rude, but it can sound quite direct depending on tone and situation. Adding por favor is very common if you want to soften it.

Yes. Spanish often softens requests by using a question or a modal expression.

For example:

• ¿Puedes hablar en voz baja? = Can you speak quietly?
• ¿Podrías hablar en voz baja? = Could you speak quietly?
• Por favor, habla en voz baja. = Please speak quietly.

These are often more natural if you are making a polite request rather than giving a direct instruction.

Yes, definitely.

It is very useful and very common, and it pairs nicely with its opposite:

• en voz baja = quietly / in a low voice
• en voz alta = out loud / in a loud voice

These are excellent chunks to learn as complete expressions rather than trying to build them from scratch every time.

Sometimes, yes, but the nuance is different.

• Habla en voz baja. = neutral, standard, very common
• Habla bajito. = more colloquial, often softer or more affectionate in tone

So bajito can work in conversation, but en voz baja is the more standard expression and the best one to learn first.