Mi sobrina quiere cortarse el flequillo antes de empezar el curso.

Because cortarse is the reflexive form, and here it means to have one’s hair cut / to cut one’s own hair.

With haircuts and similar grooming actions, Spanish often uses a reflexive verb where English does not. So:

• cortar el flequillo = to cut the fringe/bangs
• cortarse el flequillo = to cut one’s fringe/bangs / to get one’s fringe trimmed

In this sentence, Mi sobrina quiere cortarse el flequillo, the idea is that the niece wants this done to herself.

Because after a conjugated verb like quiere, Spanish allows the second verb to stay in the infinitive, and any attached pronoun can go on that infinitive.

So both of these are correct:

• Mi sobrina quiere cortarse el flequillo.
• Mi sobrina se quiere cortar el flequillo.

They mean the same thing. The first version is very common and sounds natural.

This is a general rule with verb combinations:

• voy a ducharme / me voy a duchar
• puede levantarse / se puede levantar

Spanish often uses the definite article (el, la, los, las) instead of a possessive adjective with body parts, clothing, and personal items, especially when it is already clear whose thing it is.

So Spanish prefers:

• Se lava las manos = He/She washes his/her hands
• Me duele la cabeza = My head hurts
• Quiere cortarse el flequillo = She wants to cut her fringe/bangs

Using su flequillo is not impossible, but el flequillo is more natural here because the reflexive pronoun already tells us it is her own hair.

In Spain, flequillo means the hair that falls over the forehead: fringe in British English or bangs in American English.

So:

• tener flequillo = to have a fringe / bangs
• cortarse el flequillo = to trim one’s fringe / bangs

This is a very common word in Spain.

Because the sentence is talking about the same subject for both actions.

• Mi sobrina quiere cortarse el flequillo antes de empezar el curso.

This means the niece wants to cut her fringe before she starts the course/school year. Since the person doing both actions is the same, Spanish normally uses:

• antes de + infinitive

If the subject changes, Spanish usually uses:

• antes de que + subjunctive

For example:

• Mi sobrina quiere cortarse el flequillo antes de que empiece el curso.

This version is also possible, but now the focus is more on the course beginning as a separate event. It is very natural too.
A simple way to remember it:

• same subject → antes de + infinitive
• different subject (or treated as a separate clause) → antes de que + subjunctive

In Spain, el curso often refers to the school year, academic year, or sometimes the term/course of study, depending on context.

In this sentence, antes de empezar el curso most naturally means:

• before the school year starts
• or before starting classes

A learner might expect curso to only mean a specific class, but in Spain it is very common in education-related contexts for the broader academic period too.

Spanish often uses the present tense the same way English does for current wants, intentions, or plans.

So:

• Mi sobrina quiere cortarse el flequillo... = My niece wants to cut her fringe...

It describes her current desire or intention. Nothing unusual is happening here grammatically: querer + infinitive is the standard way to say to want to do something.

Examples:

• Quiero salir. = I want to go out.
• Quiere estudiar más. = He/She wants to study more.

Because mi and mí are different words in Spanish.

• mi = my → possessive adjective
• mí = me after a preposition

So here:

• Mi sobrina = My niece

Compare:

• para mí = for me
• sin mí = without me

So in this sentence, mi has no accent because it means my.

Yes. In everyday Spanish, cortarse el pelo or cortarse el flequillo does not always mean she personally will do the cutting with scissors herself. It often simply means that she is going to get it cut.

So:

• Mi sobrina quiere cortarse el flequillo

can mean:

• she wants to trim her bangs herself, or
• she wants to have them trimmed

Context usually makes it clear. This is very common with grooming verbs in Spanish.

Yes. Spanish is flexible here. Some natural alternatives are:

• Mi sobrina quiere cortarse el flequillo antes de empezar el curso.
• Mi sobrina se quiere cortar el flequillo antes de empezar el curso.

Both are correct and natural.

You could also move the time phrase for emphasis:

• Antes de empezar el curso, mi sobrina quiere cortarse el flequillo.

The basic meaning stays the same.