Yo prefiero un traje sencillo con chaleco cuando hace frío.

Not usually. You can simply say Prefiero un traje sencillo con chaleco cuando hace frío.

Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action. Prefiero clearly means I prefer.

Yo is included only if you want to:

• add emphasis: I prefer...
• create contrast: Yo prefiero un traje sencillo, pero él prefiere algo más formal

So in this sentence, yo is optional.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos/ellas prefieren

So prefiero is the correct yo form.

This is very common in Spanish. Other verbs do the same kind of change, such as:

• querer → quiero
• pensar → pienso

Here, traje means suit as an item of clothing.

So un traje sencillo = a simple/plain suit.

A useful thing to know is that traje can also be the I brought form of traer:

• Yo traje el libro = I brought the book

But in this sentence, because it comes after un and is modified by sencillo, it clearly means the noun suit.

Sencillo usually means simple, plain, or unfussy.

With clothing, it suggests something:

• not flashy
• not overly elaborate
• neat and understated

So un traje sencillo is not necessarily a cheap suit. It is more about style than price.

Because sencillo agrees with traje, which is a masculine singular noun.

• traje = masculine singular
• so the adjective must also be masculine singular: sencillo

Compare:

• un traje sencillo
• una chaqueta sencilla

This is standard adjective agreement in Spanish.

Both are possible, but they do not sound exactly the same.

• con chaleco means with a waistcoat/vest, focusing on the type of outfit
• con un chaleco means with a waistcoat/vest, but it sounds a bit more like you are referring to one particular item

In clothing descriptions, Spanish often omits the article when talking generally about what something includes or is worn with:

• un traje con chaleco
• una camisa con corbata

So con chaleco sounds very natural here.

Yes, but for English speakers this needs a small warning.

In this context, because the sentence talks about a suit, chaleco is best understood as waistcoat in British English or vest in American English.

So:

• traje con chaleco = three-piece suit / suit with a waistcoat

Be careful, because English vest can mean different things depending on the variety of English.

Because this sentence expresses a general habit or preference:

• I prefer a simple suit with a waistcoat when it’s cold

Spanish uses the present tense after cuando for habitual or repeated situations:

• cuando hace frío
• cuando llueve
• cuando tengo tiempo

If you were talking about a specific future situation, Spanish would normally use the subjunctive:

• cuando haga frío = when it gets/is cold in the future

So:

• cuando hace frío = whenever it is cold / when it is cold in general
• cuando haga frío = when it is cold in the future

Because hacer is commonly used in weather expressions.

• hace frío = it is cold
• hace calor = it is hot
• hace buen tiempo = the weather is good

You generally do not say es frío to mean it is cold in the weather sense.

Es frío would more likely describe something or someone as cold in nature:

• El clima es frío = The climate is cold
• Es una persona fría = He/She is a cold person

So for weather in the moment or in general conditions, hace frío is the normal expression.

Sometimes, but the meaning shifts slightly.

• Prefiero = I prefer
• Me gusta más = I like ... more

So:

• Prefiero un traje sencillo con chaleco... = I prefer a simple suit with a waistcoat...
• Me gusta más un traje sencillo con chaleco... = I like a simple suit with a waistcoat more...

Prefiero is the most direct and natural choice for expressing preference.

Because traje is a singular masculine noun, and here it means a suit, not the suit.

• un traje = a suit
• el traje = the suit

So the sentence is talking about a general preference, not one specific suit.

Yes, Spanish word order is flexible, though the original version is the most neutral and natural.

Standard order:

• Yo prefiero un traje sencillo con chaleco cuando hace frío

You could also hear:

• Cuando hace frío, prefiero un traje sencillo con chaleco

That version puts more focus on the time/condition when it’s cold.

What you usually would not do without a special reason is move things around randomly, because it can sound marked or unnatural.

Frío is pronounced in two syllables: frí-o.

The written accent shows where the stress goes:

• frí-o

Without the accent, Spanish spelling rules would suggest a different pronunciation pattern.

The accent mark helps show that the í is stressed and that the word is pronounced as two syllables, not one.

Yes, it sounds natural and correct in Spain.

A speaker from Spain would understand it as something like:

• I prefer a simple suit with a waistcoat when it’s cold

If you wanted a slightly smoother, more conversational version, you might also hear:

• Cuando hace frío, prefiero un traje sencillo con chaleco
• Cuando hace frío, prefiero llevar un traje sencillo con chaleco

Adding llevar makes the idea of wearing more explicit, but your original sentence is already perfectly good.

Yes, a little.

Spanish often allows preferir + noun:

• Prefiero un traje sencillo...

This already means that this is the outfit you prefer.

But if you want to be very explicit about wearing it, you can say:

• Prefiero llevar un traje sencillo con chaleco cuando hace frío

That sounds slightly more specific:

• prefiero un traje... = I prefer a simple suit...
• prefiero llevar un traje... = I prefer to wear a simple suit...

Both are correct.