Quando faz frio, eu prefiro usar calça jeans.

In Brazilian Portuguese, fazer is very commonly used for weather and temperature expressions:

• faz frio = it’s cold
• faz calor = it’s hot
• faz sol = it’s sunny

So faz frio is the natural way to say it’s cold in this kind of sentence.

You can use frio with ser in other contexts, but the meaning is different:

• Hoje está frio. = Today it’s cold.
• Essa cidade é fria. = This city is cold. / This city has a cold climate.

So in Quando faz frio..., faz frio is the standard weather expression.

Because fazer in weather expressions is impersonal in Portuguese. That means it does not really have a normal subject.

So:

• Faz frio.
• Fazia calor.
• Vai fazer vento.

The verb stays in 3rd person singular.

English uses a dummy subject it in sentences like it’s cold, but Portuguese usually does not need that kind of subject here.

Both can be possible, but they are not always used in exactly the same way.

• Quando faz frio sounds like a general/habitual condition: whenever the weather is cold.
• Quando está frio often sounds more like the condition at a particular time or situation: when it is cold right now / when the weather feels cold.

In your sentence, Quando faz frio, eu prefiro usar calça jeans, the speaker is talking about a general habit, so faz frio fits very well.

Because the sentence expresses a general repeated situation:

• Quando faz frio, eu prefiro usar calça jeans. = Whenever it’s cold, I prefer to wear jeans.

In Portuguese, for general truths or habits, quando often takes the present indicative.

Compare that with a future idea:

• Quando fizer frio, eu vou usar calça jeans. = When it gets cold / when it is cold in the future, I’m going to wear jeans.

So:

• quando faz frio = habitual/general
• quando fizer frio = future situation

Because Quando faz frio is an introductory clause placed before the main clause:

• Quando faz frio, eu prefiro usar calça jeans.

That comma helps separate:

1. the time clause: Quando faz frio
2. the main statement: eu prefiro usar calça jeans

This is very common in Portuguese, just as in English with something like:

• When it’s cold, I prefer to wear jeans.

If the order changes, the comma is often unnecessary:

• Eu prefiro usar calça jeans quando faz frio.

Yes, it could be omitted.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear:

• Quando faz frio, prefiro usar calça jeans.

That is completely natural.

Including eu can add:

• emphasis
• contrast
• clarity

For example:

• Quando faz frio, eu prefiro usar calça jeans, mas minha irmã prefere saia.

So eu is correct, but not required.

Because preferir can be followed by an infinitive when you prefer doing an action:

• prefiro usar = I prefer to wear
• prefiro comer = I prefer to eat
• prefiro ficar em casa = I prefer to stay home

This is very similar to English prefer to + verb.

You can also prefer a noun:

• Prefiro calça jeans. = I prefer jeans.
• Prefiro café. = I prefer coffee.

But here the speaker is talking about the action of wearing, so usar is used.

Yes, but usar is often the most natural everyday verb for wearing clothes in Brazilian Portuguese.

• usar calça jeans = to wear jeans
• usar camiseta = to wear a T-shirt

Vestir also exists, but it can feel a bit more formal or can focus more on the act of dressing:

• vestir uma camisa
• vestir-se = to get dressed

In everyday Brazilian Portuguese, usar is extremely common when talking about what someone wears.

Both are possible, but they are a little different.

• calça jeans specifically means jeans pants
• jeans by itself can also mean jeans, and in many contexts that is enough

Examples:

• Prefiro usar calça jeans.
• Prefiro usar jeans.

Both sound natural.

Using calça jeans is just a bit more explicit, since jeans can describe the material/style and calça tells you it is a pair of pants.

Because in Portuguese, a single pair of pants is usually treated as a singular noun:

• uma calça = a pair of pants
• uma calça jeans = a pair of jeans

This is different from English, where pants and often jeans are grammatically plural.

So even though English says jeans or pants, Portuguese normally uses singular calça for one item.

Portuguese often omits the article when speaking in a general way about clothing, especially after verbs like usar.

So:

• prefiro usar calça jeans = I prefer to wear jeans / jeans pants

This sounds generic, meaning that kind of clothing in general.

If you add an article, the meaning can become more specific:

• prefiro usar uma calça jeans = I prefer to wear a pair of jeans

That is also correct, but it points more to a pair rather than the clothing type in general.

Yes. Portuguese allows flexible word order here.

For example:

• Quando faz frio, eu prefiro usar calça jeans.
• Eu prefiro usar calça jeans quando faz frio.

Both are correct.

The first version emphasizes the condition when it’s cold first.
The second starts with the main idea I prefer to wear jeans.

Usually, for one pair, Brazilian Portuguese commonly uses calça jeans in the singular:

• uma calça jeans

If you mean multiple pairs, then plural makes sense:

• duas calças jeans

So in your sentence, calça jeans is natural because it refers to the kind of clothing the speaker prefers to wear, not to several pairs.

It normally sounds like a habit or general preference.

• Quando faz frio = whenever it’s cold
• eu prefiro usar calça jeans = I prefer to wear jeans

So the whole sentence means something like:

• whenever cold weather happens, this is my usual preference

If you wanted to talk about a specific moment, Portuguese would often use different context or wording, such as:

• Agora que está frio, eu prefiro usar calça jeans. = Now that it’s cold, I prefer to wear jeans.

Yes. Some common alternatives are:

• Gosto mais de usar calça jeans quando faz frio. = I like wearing jeans more when it’s cold.
• Quando faz frio, gosto de usar calça jeans. = When it’s cold, I like to wear jeans.
• Quando faz frio, costumo usar calça jeans. = When it’s cold, I usually wear jeans.

But prefiro usar is a very direct and natural way to express preference.