Mi hermana quiere mudarse a otro piso antes de que termine el semestre.

Because mudarse is the usual verb for to move house / to move to a new home in Spanish.

• mudar by itself exists, but for everyday Spanish about changing where you live, mudarse is much more natural.
• The reflexive pronoun is attached to the infinitive because it comes after quiere:

quiere mudarse = wants to move

If you conjugate it, the pronoun changes:

• Me mudo
• Te mudas
• Se muda

So Mi hermana quiere mudarse literally has the idea of My sister wants to move herself, but in natural English that is just My sister wants to move.

In Spanish, when you have a conjugated verb followed by an infinitive, a reflexive pronoun can either:

• go before the conjugated verb, or
• attach to the infinitive

With querer + infinitive, both are possible:

• Mi hermana se quiere mudar
• Mi hermana quiere mudarse

Both are correct. In your sentence, the pronoun is attached to mudarse, which is very common.

What you cannot do is say quiere se mudar. The pronoun does not go in the middle like that.

In Spain, piso usually means flat / apartment.

So mudarse a otro piso means to move to another flat/apartment.

This is worth noticing because in many Latin American varieties, piso more often means floor or storey, and apartamento, departamento, or casa may be more common for the home itself.

So this sentence sounds very natural for Spain Spanish.

Because otro normally does not combine with un.

So Spanish says:

• otro piso = another flat
• not un otro piso

This is similar to how English says another flat, not one another flat.

Also, otro already works as a determiner here, so no extra indefinite article is needed.

Because antes changes structure depending on what comes after it.

1. Before a noun or infinitive:

Use antes de

• antes del semestre
• antes de mudarse

2. Before a full clause with a new verb:

Use antes de que

• antes de que termine el semestre

That second part has its own verb, termine, so Spanish needs de que.

Because antes de que normally triggers the subjunctive.

Spanish uses the subjunctive after expressions like antes de que when the action is seen as:

• pending,
• not yet completed,
• or anticipated rather than stated as a fact.

Here, the semester has not ended yet at the moment we are talking about, so Spanish says:

• antes de que termine el semestre

not

• antes de que termina el semestre

The subjunctive is very common after time expressions when the action is still in the future or not yet realized.

Because termine is the present subjunctive form of terminar, while termina is the present indicative.

For -ar verbs, the present subjunctive endings are:

• yo termine
• tú termines
• él/ella termine
• nosotros terminemos
• vosotros terminéis
• ellos terminen

Since the subject here is el semestre, Spanish needs the él/ella/usted form:

• termine

So the choice is grammatical, not optional: antes de que calls for the subjunctive.

Because terminar here is not reflexive.

The subject is el semestre, and the verb simply means to end:

• el semestre termina = the semester ends

So in the clause antes de que termine el semestre, there is no reflexive meaning and no reflexive pronoun needed.

The se only belongs to mudarse, because that verb is reflexive.

Yes, Spanish could leave it out if the meaning were clear from context:

• antes de que termine

However, including el semestre makes the sentence clearer and more explicit. Spanish often drops subjects when they are pronouns, but with nouns like el semestre, keeping them is very normal.

Also, here it helps avoid any doubt about what is ending.

Not in this exact structure, because the subject of the second action is different.

Compare:

Same subject -> infinitive is possible

• Mi hermana quiere mudarse antes de terminar el semestre

This sounds like before finishing the semester herself

Different or explicit clause -> antes de que + subjunctive

• Mi hermana quiere mudarse antes de que termine el semestre

This means before the semester ends

So the original sentence is not about your sister finishing something; it is about the semester ending.

It is nonspecific.

Otro piso means another flat, without identifying which one. If the speaker meant a specific flat already known to both people, Spanish might use something more definite, depending on context, such as:

• el otro piso

But in your sentence, otro piso just means some different flat from the current one.

Not here. The standard and correct form in this structure is antes de que.

So you say:

• antes de que termine el semestre

not

• antes que termine el semestre

You may sometimes hear antes que in other contexts or regional uses, but for learners, antes de que + subjunctive is the reliable pattern to use.

The structure is:

Mi hermana + quiere + mudarse a otro piso + antes de que termine el semestre

Broken down:

• Mi hermana = subject
• quiere = main verb
• mudarse = infinitive complement of quiere
• a otro piso = destination / place she wants to move to
• antes de que termine el semestre = time clause

So the sentence is built around querer + infinitive, followed by a time clause with antes de que + subjunctive.