Mi hermana prefiere hacerse un moño cuando hace calor.

Because preferir is the infinitive, meaning to prefer. In the sentence, the verb has to be conjugated to match the subject mi hermana.

So:

• preferir = to prefer
• mi hermana prefiere = my sister prefers

Also, preferir is a stem-changing verb in the present tense:

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

Notice the change from e to ie in most forms.

Hacerse is the reflexive form of hacer here, and it means something like to do one’s hair into or to make oneself a hairstyle.

So:

• hacer un moño = to make a bun
• hacerse un moño = to put one’s hair up in a bun / to make oneself a bun

The reflexive pronoun se shows that the person is doing it to herself.

In this sentence, your sister is not making a bun for someone else; she is styling her own hair. That is why hacerse is used.

Because hacerse is an infinitive after a conjugated verb: prefiere hacerse.

In Spanish, when you have a conjugated verb plus an infinitive, reflexive pronouns can attach to the infinitive:

• prefiere hacerse un moño

You could not put it before prefiere here and say se prefiere hacer un moño, because that would change the structure and sound wrong for this meaning.

The idea is:

• prefiere = prefers
• hacerse un moño = to make herself a bun

Together: She prefers to make herself a bun.

In this sentence, un moño means a bun, meaning a hairstyle where the hair is gathered and twisted or tied up.

In Spain, moño commonly refers to that hairstyle.

Depending on context, moño can also mean other things in Spanish, such as a bow or ribbon bow, but here the context clearly shows it means a hair bun because of hacerse and cuando hace calor.

Spanish usually uses an article in expressions like this when talking about a hairstyle or physical arrangement:

• hacerse un moño
• llevar una coleta
• tener una barba larga

So un moño means a bun.
It sounds natural in Spanish to include the article here.

English sometimes drops words in similar expressions, but Spanish normally does not.

Cuando means when, and hace calor is a very common Spanish expression meaning it is hot or the weather is hot.

Literally, hace calor is something like it makes heat, but you should learn it as a set expression.

Spanish often uses hacer for weather-related expressions:

• hace calor = it’s hot
• hace frío = it’s cold
• hace viento = it’s windy
• hace sol = it’s sunny

So cuando hace calor means when it’s hot.

They are related to the same verb hacer, but they are being used in two different ways:

1. hacerse un moño
   Here, hacerse means to make oneself / to do one’s hair into a bun.

2. hace calor
   Here, hace is part of the weather expression hace calor = it is hot.

So although both come from hacer, they do different jobs in the sentence.

Not in this sentence if you want the same meaning.

There is an important difference:

• hace calor = the weather is hot / it’s hot
• tener calor = to feel hot

So:

• cuando hace calor = when the weather is hot
• cuando tiene calor = when she feels hot

Both are possible in general, but they mean different things.

This sentence is talking about hot weather, not just your sister’s physical sensation.

Spanish often leaves the subject out because the verb ending already shows who the subject is:

• Prefiere hacerse un moño...

That would be grammatically fine if the context already made it clear who you are talking about.

But mi hermana is included here to make the subject explicit. It adds clarity and emphasis.

So both are possible depending on context:

• Mi hermana prefiere hacerse un moño...
• Prefiere hacerse un moño...

The first is clearer if there is no previous context.

Because mi without an accent is the possessive adjective meaning my:

• mi hermana = my sister

Mí with an accent is a pronoun used after prepositions:

• para mí = for me
• a mí = to me

So here you need mi, because it is directly describing the noun hermana.

Yes. Spanish is fairly flexible with word order, especially with time clauses like cuando hace calor.

You can say:

• Mi hermana prefiere hacerse un moño cuando hace calor.
• Cuando hace calor, mi hermana prefiere hacerse un moño.

Both are correct.

The second version puts more emphasis on the condition when it’s hot.

No. Moño can have more than one meaning depending on context.

Common meanings include:

• hair bun
• decorative bow

But in this sentence, it clearly means hair bun, because:

• it follows hacerse,
• the subject is a person doing something to herself,
• and the context is when it’s hot, which fits a hairstyle.

So learners should remember that the context decides the meaning.

Yes, it is a very natural expression in Spain for putting your hair up in a bun.

Spanish can vary by region, and some areas may use slightly different expressions or words for hairstyles, but hacerse un moño is standard and widely understood.

So for Spanish from Spain, this is a very useful phrase to know.

Because there is no direct object referring to a specific person.

The personal a is used before a direct object that is a specific person:

• Veo a mi hermana. = I see my sister.

But here:

• mi hermana is the subject
• un moño is a thing, not a person

So no personal a is needed.

It is in the present tense:

• prefiere = prefers
• hace = it is / it makes

The sentence expresses a general habit or preference, not something happening only right now.

So it means something like a usual pattern:

• My sister prefers to wear her hair in a bun when it’s hot.

That is a very common use of the Spanish present tense.