Cuando termine el semestre, quiero cortarme el pelo.

Because after cuando referring to a future event, Spanish normally uses the present subjunctive.

So:

• Cuando termine el semestre... = When the semester ends / when the semester is over...

Even though this is about the future, Spanish does not usually say cuando termina here.

Compare:

• Cuando termina el semestre, siempre estoy cansado.
  = When the semester ends, I’m always tired.
  This is a habitual/repeated situation, so indicative is used.

• Cuando termine el semestre, quiero cortarme el pelo.
  = a specific future situation, so subjunctive is used.

Because quiero describes your present desire or intention.

The sentence means something like:

• Right now, I want to get my hair cut when the semester ends.

Spanish often uses the present tense this way:

• Quiero hacerlo mañana. = I want to do it tomorrow.
• Espero verte la semana que viene. = I hope to see you next week.

So the wanting is now, even if the action happens later.

Cortarme is made of:

• cortar = to cut
• me = myself / to me (reflexive pronoun)

So cortarme el pelo literally looks like to cut myself the hair, but in natural English it means:

• to cut my hair
• very often to get my hair cut

This is a very common Spanish structure with personal grooming actions.

Because when a reflexive pronoun goes with an infinitive, it can be attached to the end.

So these are both correct:

• Quiero cortarme el pelo.
• Me quiero cortar el pelo.

Both mean the same thing. The first one is very natural and common.

This also happens with other infinitives:

• Voy a lavarme las manos.
• Te puedes sentar.
• Necesito ducharme.

With body parts and many personal care actions, Spanish usually uses the definite article (el, la, los, las) rather than a possessive like mi.

So Spanish prefers:

• Me lavo las manos. = I wash my hands.
• Se cepilla los dientes. = He/She brushes his/her teeth.
• Quiero cortarme el pelo. = I want to cut/get my hair cut.

The ownership is already clear from me, so mi is usually unnecessary.

You can say mi pelo, but it usually adds emphasis, contrast, or a special context.

Usually, in normal everyday Spanish, cortarse el pelo means to get one’s hair cut or simply to have a haircut.

It does not necessarily mean you will personally do the cutting yourself.

So:

• Quiero cortarme el pelo.
  usually means I want to get my hair cut.

If you want to make it very clear that you will cut it yourself, you would usually add more context, for example:

• Quiero cortarme el pelo yo mismo / yo sola.

The reflexive changes whose hair is involved.

• cortar el pelo = to cut hair / to cut someone’s hair
• cortarse el pelo = to cut one’s own hair / more commonly to get one’s hair cut

Examples:

• El peluquero corta el pelo.
  = The hairdresser cuts hair.

• Quiero cortarme el pelo.
  = I want to get my hair cut.

So cortarse points back to the person affected.

Yes. It depends on whether the action is viewed as:

• future/unknown/not yet completed → usually subjunctive
• habitual, past, or already-known fact → usually indicative

Examples:

• Cuando termine el semestre, viajaré.
  Future event → subjunctive

• Cuando termina el semestre, todo el mundo sale de fiesta.
  Habitual event → indicative

• Cuando terminó el semestre, me fui de viaje.
  Past completed event → indicative

This is one of the most important uses of the subjunctive in Spanish.

Yes, you might hear that, but it is slightly different in structure.

• Cuando termine el semestre = When the semester ends
• Cuando se termine el semestre = When the semester finishes / when the semester is over

The version with se can make it sound a bit more like the semester comes to an end rather than focusing on some agent or simple active verb. In many contexts, both work naturally.

That said, Cuando termine el semestre is very straightforward and completely normal.

It is perfectly correct, and people in Spain do use semestre, especially in university or administrative contexts.

However, depending on the educational context, you might also hear other words such as:

• curso = school year / academic year
• trimestre = term of three months
• cuatrimestre = four-month term
• finales = final exams / exam period, in some contexts

So semestre is correct, but the most natural word can depend on how that particular school or university organizes the year.

Yes. Spanish allows some flexibility.

These are all possible:

• Cuando termine el semestre, quiero cortarme el pelo.
• Quiero cortarme el pelo cuando termine el semestre.
• Me quiero cortar el pelo cuando termine el semestre.

They all mean basically the same thing. The difference is mostly about rhythm, emphasis, or what you want to highlight first.

The original sentence is very natural and clear.