Por la noche prefiero relajarme en la bañera cuando hace frío.

In Spanish, por la noche is the normal way to say at night or in the evening/at night.

• por la mañana = in the morning
• por la tarde = in the afternoon / evening
• por la noche = at night

En la noche does exist, but it is much less common in Spain for this general meaning. It sounds more like during the night in a specific context.

So for a general routine or preference, por la noche is the natural choice.

It often means at night / in the evenings in a general sense, especially when talking about habits or preferences.

So in this sentence, it can sound like a general preference: At night, I prefer to relax in the bathtub when it’s cold.

If you want to make the habitual meaning even clearer, Spanish often uses por las noches:

• Por las noches prefiero relajarme en la bañera cuando hace frío.

That sounds very natural too and makes the repeated habit more obvious.

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

• prefiero = I prefer

So yo prefiero is possible, but usually unnecessary unless you want emphasis or contrast:

• Yo prefiero relajarme, pero mi hermano prefiere salir.
  = I prefer to relax, but my brother prefers to go out.

In your sentence, leaving out yo is completely normal.

Yes. Preferir is a stem-changing verb.

Its 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.

After preferir, Spanish usually uses an infinitive when you are talking about preferring to do an action.

• Prefiero relajarme = I prefer to relax
• Prefiero leer = I prefer to read

This works much like English prefer to + verb.

You can also prefer a thing:

• Prefiero el té = I prefer tea

Relajarme means to relax or more literally to relax myself.

The base verb is relajarse, a reflexive verb:

• relajarse = to relax, to unwind

When a reflexive verb is used with an infinitive, the reflexive pronoun can be attached to the end:

• prefiero relajarme

That me refers back to the speaker: myself.

Compare:

• Quiero acostarme = I want to go to bed
• Necesito ducharme = I need to shower

No, that would be incorrect.

With an infinitive, the reflexive pronoun normally stays attached to the infinitive:

• prefiero relajarme

If the verb were conjugated, then the pronoun would go before it:

• me relajo = I relax
• prefiero relajarme = I prefer to relax

So the reflexive pronoun belongs to relajarse, not to preferir.

En la bañera means in the bathtub.

• en = in
• la bañera = the bathtub

It is used because the person is relaxing in that place.

You would not use a la bañera here, because a usually suggests movement toward something:

• Voy a la bañera = I go to the bathtub
• Estoy en la bañera = I am in the bathtub

In your sentence, the idea is location, so en is correct.

They are different things:

• la bañera = the bathtub
• el baño = the bathroom

So:

• en la bañera = in the bathtub
• en el baño = in the bathroom

That is an important difference, because English learners sometimes confuse baño with bath. In Spanish, baño usually means the room, not the tub itself.

Yes. In Spain, bañera is the standard word for bathtub.

In some parts of Latin America, people may also say tina, but in Spain bañera is the normal and expected choice.

Spanish often uses hacer for weather expressions.

• hace frío = it’s cold
• hace calor = it’s hot
• hace buen tiempo = the weather is nice
• hace mal tiempo = the weather is bad

This is an impersonal expression. There is no real subject like English it.

So cuando hace frío literally means something like when it makes cold, but in natural English it means when it’s cold.

Because está frío usually describes a thing being cold, not the weather.

For example:

• La sopa está fría = The soup is cold
• El agua está fría = The water is cold

But for weather, Spanish normally says:

• Hace frío = It’s cold

So in your sentence, cuando hace frío is the correct weather expression.

Both are possible, but they are slightly different.

• cuando hace frío = when it’s cold
• si hace frío = if it’s cold

Cuando sounds like this is a situation that regularly happens or a condition that naturally comes up.
Si sounds more conditional or uncertain.

In your sentence, cuando hace frío fits well because it expresses a general situation in which the speaker likes to relax in the bathtub.

Yes, both are possible.

• Por la noche prefiero relajarme en la bañera cuando hace frío.
• Prefiero relajarme en la bañera por la noche cuando hace frío.

Starting with por la noche puts the time expression first, which can make the sentence feel more natural or set the scene first: At night, I prefer...

Spanish word order is flexible, so both versions are grammatical. The original just gives a little prominence to the time phrase.

Yes, it is natural and clear.

A speaker from Spain might also say:

• Por las noches prefiero relajarme en la bañera cuando hace frío.

That version can sound even more natural if you mean a general habit. But the original sentence is perfectly good Spanish.