La bañera tarda en llenarse cuando el agua cae despacio.

Because tardar normally uses the pattern tardar en + infinitive.

• tardar en hacer algo = to take a while to do something
• La bañera tarda en llenarse = The bathtub takes a while to fill up

So the en is part of the standard structure after tardar.

More examples:

• Tardo en entenderlo = I take a while to understand it.
• El tren tarda en llegar = The train takes a while to arrive.

Without en, it sounds non-standard or wrong in this kind of sentence.

Because llenarse means to fill up / to become full, while llenar usually means to fill something.

Compare:

• llenar la bañera = to fill the bathtub
• la bañera se llena = the bathtub fills up

In La bañera tarda en llenarse, the bathtub is the thing that becomes full, so Spanish uses the reflexive form llenarse.

You can think of it like this:

• llenar = to fill
• llenarse = to fill up / to get filled

This reflexive form is very common in Spanish with changes of state:

• romper = to break something
• romperse = to break
• abrir = to open something
• abrirse = to open

Because when a reflexive verb is in the infinitive, the pronoun can be attached to the end.

So:

• llenarse = llenar + se

This is completely normal with infinitives:

• voy a lavarme
• quiero sentarme
• empieza a moverse

You could also sometimes place the pronoun before a conjugated verb in a larger structure, but here the normal form is simply en llenarse.

Because this sentence expresses a general truth or a habitual situation.

When cuando means when/whenever and refers to something that generally happens, Spanish uses the present indicative:

• Cuando llueve, me quedo en casa = When it rains, I stay home.
• Cuando el agua cae despacio, la bañera tarda en llenarse = When the water runs/falls slowly, the bathtub takes longer to fill.

Spanish would use the subjunctive after cuando if the action is in the future or has not happened yet:

• Cuando el agua caiga despacio, la bañera tardará en llenarse = When the water falls slowly, the bathtub will take longer to fill.

So in your sentence, the present tense is used because it describes a normal repeated fact.

Yes, caer literally means to fall, but with water Spanish often uses it in a broader way than English.

So el agua cae despacio can mean:

• the water falls slowly
• the water comes down slowly
• the water runs slowly

In natural English, you might not say falls here, but in Spanish caer sounds normal when talking about water coming down from a tap, shower, rain, etc.

Related examples:

• La lluvia cae fuerte = The rain is falling hard.
• El agua cae de la ducha = The water comes down from the shower.

Depending on context, Spanish could also use other verbs, but cae is perfectly natural here.

Possibly, but it changes the image slightly.

• el agua cae despacio focuses on the water falling/coming down
• el agua sale despacio focuses on the water coming out

If you are thinking about the water leaving the tap, sale can work. If you are thinking about the stream of water dropping into the bathtub, cae works very well.

So both can make sense, but cae is not a strange choice.

Spanish often uses the definite article more than English does, especially when talking about something in a general or understood way.

Here la bañera can mean:

• the bathtub in the situation being discussed
• the bathtub as a general thing

English often prefers the bathtub too, so this is not very unusual here.

Compare:

• La bañera tarda en llenarse = The bathtub takes a while to fill.
• Una bañera tarda en llenarse = A bathtub takes a while to fill.

The version with una sounds more like a general statement about any bathtub, while la can sound more like the bathtub in question or the idea of a bathtub in general.

No.

• bañera = bathtub
• baño = bathroom, bath, or act of bathing depending on context

So in this sentence:

• La bañera = the bathtub

Examples:

• El baño está al fondo = The bathroom is at the end.
• Me doy un baño = I take a bath.
• La bañera es grande = The bathtub is big.

This is a very common vocabulary distinction for learners.

Both are possible, but despacio is very common and natural in everyday speech.

• despacio = slowly
• lentamente = slowly

The difference is mostly one of style and frequency:

• despacio often sounds more conversational
• lentamente can sound a bit more formal or descriptive

So:

• el agua cae despacio
• el agua cae lentamente

Both are correct.

It is an adverb here, because it modifies the verb cae.

It tells you how the water falls:

• cae despacio = falls slowly

You would not change despacio for gender or number:

• not despacia
• not despacios

It always stays despacio.

Yes. Spanish allows more flexibility than English.

You could also say:

• Cuando el agua cae despacio, la bañera tarda en llenarse.

That means the same thing. The difference is mainly one of emphasis or rhythm.

In your original sentence, the main point comes first:

• La bañera tarda en llenarse...

In the reordered version, the condition comes first:

• Cuando el agua cae despacio...

Both are correct and natural.

A simple guide:

• bañera ≈ ban-YEH-ra, but with ñ pronounced like the ny in canyon
• llenarse in most of Spain is usually pronounced roughly like ye-NAR-se

Two key sounds:

• ñ = like ny
  • bañera
• ll in most modern Spanish accents sounds like y
  • llenarse ≈ yenarse

Also:

• cae is two syllables: CA-e
• despacio in Spain is often pronounced with a th sound for the c before i:
  • des-PA-thyo in much of Spain
  • in Latin American pronunciation, more like des-PA-syo

Yes. That is a very natural English rendering.

Spanish tarda en llenarse does not always need to be translated word-for-word as takes time to fill. Depending on context, natural English could be:

• The bathtub takes a while to fill when the water runs slowly.
• The bathtub takes longer to fill when the water runs slowly.
• The tub fills slowly when the water comes out slowly.

So even though the Spanish structure is fixed, the best English translation may vary a bit depending on style and context.