Dopo tre vasche, mi fermo perché ho freddo.

Literally, vasche means tubs or pools, from the singular vasca.

But in swimming, fare una vasca commonly means to swim one length of the pool. So:

• dopo tre vasche = after three lengths
• depending on context, English might also say after three laps

This is a very natural Italian swimming expression.

Because in Italian, when you give a number directly before a noun, you normally do not use an article.

So you say:

• tre vasche = three lengths
• due minuti = two minutes
• quattro libri = four books

Adding an article here would sound wrong.

Because the verb here is fermarsi, which often means to stop in the sense of to stop oneself / to come to a stop.

So:

• mi fermo = I stop
• ti fermi = you stop
• si ferma = he/she stops

The mi is the reflexive pronoun that goes with fermarsi.

Italian often uses a reflexive form where English just uses a normal verb. So although English says I stop, Italian commonly says mi fermo.

The infinitive is fermarsi.

Here is the present tense:

• io mi fermo
• tu ti fermi
• lui/lei si ferma
• noi ci fermiamo
• voi vi fermate
• loro si fermano

In your sentence, mi fermo is the first person singular: I stop.

Because Italian expresses sensations like being cold, being hot, being hungry, and being thirsty with avere (to have), not essere (to be).

So:

• ho freddo = I am cold
• ho caldo = I am hot
• ho fame = I am hungry
• ho sete = I am thirsty

If you say sono freddo, it usually means I am a cold person/object or my manner is cold, not I feel cold.

Here, perché means because.

So:

• mi fermo perché ho freddo = I stop because I’m cold

The same word perché can also mean why in questions:

• Perché ti fermi? = Why are you stopping?

So Italian uses perché for both because and why; the sentence structure tells you which meaning it has.

The accent is part of the correct spelling: perché.

It marks the stress on the final syllable: per-CHÉ.

This is common in Italian words stressed on the last syllable, especially certain conjunctions and adverbs, for example:

• perché
• città
• là
• già

So yes, you should write the accent.

Because Italian often uses the present tense in the same situations where English also uses the present:

• mi fermo = I stop / I’m stopping
• ho freddo = I’m cold

This sentence sounds like a general statement or something happening right now. Italian does not need a special progressive form here.

If needed, Italian can use stare + gerundio for an ongoing action, but it is not necessary in this sentence.

Yes, but it changes the feel slightly.

• mi fermo = I stop / I’m stopping
• mi sto fermando = I am in the process of stopping

In most cases, mi fermo is the more natural and simpler choice here.
Mi sto fermando puts extra emphasis on the action happening right now.

It is the most natural and common choice.

You could also find other conjunctions such as:

• poiché
• siccome

But they are different in tone or structure:

• Mi fermo perché ho freddo = the most normal, everyday phrasing
• Siccome ho freddo, mi fermo = Since I’m cold, I stop
• Poiché ho freddo, mi fermo = more formal/literary

For everyday spoken Italian, perché is usually best.

Yes. Italian word order is flexible, though some versions sound more natural than others.

Your sentence:

• Dopo tre vasche, mi fermo perché ho freddo.

A natural variation would be:

• Mi fermo dopo tre vasche perché ho freddo.

Both are correct. Starting with Dopo tre vasche puts more focus on after three lengths.

Yes. The singular is la vasca, so it is feminine.

That gives:

• singular: la vasca
• plural: le vasche

The ending changes like this:

• -a → -e
• vasca → vasche

Also notice the spelling: the h stays in vasche to keep the hard k sound before e.

It is pronounced roughly VAH-skeh.

The important point is that ch before e or i in Italian gives a hard k sound:

• vasche = VAH-skeh
• not VAH-she

The same happens in words like:

• che
• amiche
• banca / banche

So the h is there to preserve the hard sound.