Faccio una vasca e poi torno nello spogliatoio.

Because Italian usually doesn’t need the subject pronoun when the verb ending already shows who is doing the action.

• faccio = I do / I make
• torno = I return / I go back

So io is optional here. You could say Io faccio una vasca..., but it would usually add emphasis, like I do a lap...

Because fare una vasca is a very common Italian expression in a swimming context.

Literally, fare means to do / to make, but in this expression:

• fare una vasca = to swim a length / do a lap

So Italians often say faccio una vasca where English would naturally say I swim a lap or I do a length.

You can use nuotare, but fare una vasca is more idiomatic for one pool length.

On its own, vasca can mean things like:

• bathtub
• tank
• pool basin

But in swimming language, una vasca means:

• one length
• one lap of the pool

So the meaning depends on context. In this sentence, it clearly refers to swimming.

Because vasca is being used as a countable noun here: one lap, one length.

So:

• una vasca = one lap / one length
• due vasche = two laps / two lengths

Italian normally uses an article with singular countable nouns like this.

Both are in the present indicative:

• faccio = present of fare
• torno = present of tornare

Italian uses the present tense in several ways that English might express differently:

• for a habitual action: I do a lap and then go back
• for a sequence of actions in narration
• sometimes even for a near-future sense, depending on context

So this present tense is completely natural.

Yes, poi torno would also be possible.

Here:

• e = and
• poi = then / afterwards

So e poi means and then. It connects the two actions smoothly:

1. Faccio una vasca
2. e poi torno nello spogliatoio

Using both words makes the sequence feel clear and natural.

Because tornare means to return / to go back, while andare means to go.

So:

• vado nello spogliatoio = I go into the changing room
• torno nello spogliatoio = I go back into the changing room

The sentence wants the idea of returning, not just going there for the first time.

Because nello is the contraction of:

• in + lo = nello

And spogliatoio takes the article lo, not il, because it starts with sp- followed by a consonant.

So:

• lo spogliatoio
• nello spogliatoio

This is the same pattern as:

• lo studente → nello studio? No: nello studio
• lo specchio → nello specchio

Because Italian uses lo before certain masculine singular nouns, including ones that begin with:

• s + consonant: spogliatoio, studente
• z: zaino
• ps, gn, x, and some others

So it is:

• lo spogliatoio
• not il spogliatoio

That is why in + lo becomes nello.

It can correspond to both, depending on context.

Spogliatoio is the room where people:

• change clothes
• leave their things
• often use lockers or benches

So in English, the best translation may be:

• changing room
• locker room

At a pool or gym, either can work depending on the variety of English you are using.

Yes, but there is a nuance.

• torno nello spogliatoio suggests going back inside the changing room
• torno allo spogliatoio can sound more like returning to the changing room as a destination

In many situations, both are understandable. But nello spogliatoio is especially natural if the idea is that you physically re-enter the room.

Usually una vasca is one specific length/lap.

If you want to talk more generally, you might say things like:

• faccio due vasche = I do two lengths
• vado a nuotare = I’m going swimming
• faccio qualche vasca = I do a few lengths

So fare una vasca is fairly specific, not just a general to swim.