Quando faz calor, eu gosto de nadar na piscina.

In Brazilian Portuguese, fazer is commonly used in impersonal weather expressions:

• faz calor = it is hot
• faz frio = it is cold
• faz sol = it is sunny

This faz does not mean makes in a literal sense here. It is just the normal pattern Portuguese uses for certain weather conditions.

So:

• Quando faz calor = When it’s hot

Using é calor would sound unnatural in this context.

Faz here is an impersonal verb form. That means it does not have a real subject.

Portuguese often uses the 3rd person singular for impersonal expressions:

• Faz calor
• Faz frio
• Tem muita gente
• Há um problema

So even though there is no visible subject, the verb still appears as faz, the 3rd person singular form of fazer.

Yes, in Brazil, Quando está calor is also natural.

There is a small nuance:

• Quando faz calor = a very standard, common weather expression
• Quando está calor = also common, focusing a bit more on the current condition

In many everyday situations, both work well. For a learner, faz calor is an excellent pattern to know because it is extremely common.

The comma separates the introductory clause from the main clause.

• Quando faz calor, eu gosto de nadar na piscina.

The first part, Quando faz calor, sets the time or condition: When it’s hot.
The second part is the main idea: eu gosto de nadar na piscina.

In English, we also often use a comma after an introductory clause:

• When it’s hot, I like to swim in the pool.

So the punctuation works very similarly here.

Yes, you can absolutely omit eu:

• Quando faz calor, gosto de nadar na piscina.

Brazilian Portuguese often drops subject pronouns when the meaning is clear from the verb form. Since gosto clearly means I like, eu is not required.

Including eu can:

• add emphasis
• make the sentence clearer
• sound a little more explicit

So both are correct:

• Quando faz calor, eu gosto de nadar na piscina.
• Quando faz calor, gosto de nadar na piscina.

Because gostar normally requires the preposition de.

The pattern is:

• gostar de + noun
• gostar de + infinitive

Examples:

• Eu gosto de música.
• Eu gosto de café.
• Eu gosto de nadar.
• Eu gosto de correr.

So gosto de nadar literally follows the normal grammar of gostar. You should not say gosto nadar.

After gostar de, Portuguese uses the infinitive when talking about liking an activity in general.

• Eu gosto de nadar. = I like swimming / I like to swim.

If you said eu nado, that means I swim, which is a complete statement, not the activity after like.

Compare:

• Eu gosto de nadar. = I like to swim.
• Eu nado na piscina. = I swim in the pool.

So nadar is used because it names the activity itself.

Because na is the contraction of:

• em + a = na

So:

• na piscina = in the pool / at the pool

Portuguese uses contractions very regularly with prepositions and definite articles.

Some common ones:

• de + o = do
• de + a = da
• em + o = no
• em + a = na

So nadar na piscina is the normal form, not nadar em a piscina.

Portuguese uses definite articles more often than English does.

In this sentence, na piscina can sound natural even when speaking generally about pool swimming. It does not always mean one very specific pool in the same strong way the pool might in English.

Portuguese often prefers:

• na praia = at the beach
• no parque = in the park
• na piscina = in the pool

So this is a very normal Portuguese structure, even when English might choose a slightly different article or none at all in some contexts.

Usually it most directly means in the pool, because nadar suggests being in the water.

So:

• nadar na piscina = to swim in the pool

But in some contexts, Portuguese location phrases with em + article can be translated more flexibly into English. Here, though, in the pool is the best match.

Yes, that is also correct.

Both are natural:

• Quando faz calor, eu gosto de nadar na piscina.
• Eu gosto de nadar na piscina quando faz calor.

The difference is mostly about focus and rhythm:

• Quando faz calor, ... emphasizes the condition first.
• Eu gosto de nadar na piscina... starts with the main statement.

Portuguese allows this kind of flexibility, just like English does.

Yes, calor is a noun. Literally, it means heat.

But in the expression faz calor, Portuguese normally does not use an article. This is a fixed weather expression.

So you say:

• Faz calor
• Faz frio

not usually:

• Faz o calor
• Faz o frio

Even though calor is a noun, the whole expression works like a standard weather phrase meaning it is hot.

It normally sounds like a general habit or repeated preference:

• Quando faz calor, eu gosto de nadar na piscina.
• Whenever it’s hot, I like to swim in the pool.

That is because:

• quando can mean when/whenever depending on context
• gosto expresses a general preference
• the infinitive nadar refers to the activity in general

So the sentence is not just about one single hot day. It usually describes what the speaker tends to like doing in hot weather.