Mi hermana se hizo un moño rápido antes de salir de casa.

Because hacerse is the natural verb here when someone does something to themselves, especially with hairstyles or appearance.

• hacer = to make, to do
• hacerse = to make/do something to oneself or for oneself

So:

• hizo un moño could sound like she made a bow/object
• se hizo un moño means she put her hair up in a bun / made herself a bun

In English, this is close to she did her hair in a bun or she made herself a bun.

Hizo is the preterite of hacer, third person singular.

It is used because the sentence describes a completed action in the past:

• Mi hermana se hizo un moño... = My sister put her hair in a bun...
• ...antes de salir de casa = ...before leaving home

The preterite fits well because this is a single finished action that happened before another action.

In this sentence, moño means a hair bun.

In Spain, hacerse un moño is a common way to say someone put their hair up in a bun.

Be aware that moño can have other meanings in different contexts or regions, such as a bow or decorative knot, but here the hairstyle meaning is the natural one.

Un moño is used because the sentence is introducing the hairstyle as one bun / a bun, not referring to a specific previously mentioned bun.

• un moño = a bun
• el moño = the bun
• su moño = her bun

Spanish often avoids possessives when it is already obvious whose body, hair, or clothing is involved. Since se hizo already tells us your sister did it to herself, su is unnecessary.

So se hizo un moño sounds natural.

Here, rápido is best understood as an adjective describing moño.

So:

• un moño rápido = a quick/simple bun, a bun done in a quick way

It is not mainly functioning like quickly here. If you wanted to emphasize the speed of the action itself, Spanish would more clearly say something like:

• se hizo un moño rápidamente
• se hizo un moño muy deprisa

In the original sentence, the idea is more like a quick bun than she quickly made a bun.

Because in Spanish, descriptive adjectives usually come after the noun.

So:

• un moño rápido
• una casa grande
• un café caliente

That is the normal position. English often puts adjectives before the noun, but Spanish usually places them after it.

Because Spanish uses antes de + infinitive to mean before doing something.

So:

• antes de salir = before leaving
• antes de comer = before eating
• antes de dormir = before sleeping

You cannot normally say antes salir.

Because after antes de, Spanish uses the infinitive when the subject is the same as the subject of the main clause, or when the subject is understood generally.

Here, the sister is the one doing both actions:

• she made herself a bun
• she left home

So antes de salir is the correct structure.

If the subject changed, Spanish would usually use antes de que + subjunctive:

• Mi hermana se hizo un moño antes de que saliéramos.
• My sister made herself a bun before we left.

Because salir de casa is a very common idiomatic expression meaning to leave home.

When Spanish talks about places like home, it often omits the article in fixed expressions:

• estar en casa = to be at home
• llegar a casa = to get home
• salir de casa = to leave home

If you said de la casa, it would sound more like you are referring to the physical building in a more specific way: out of the house.

Yes. That is another very natural way to express the same basic idea.

• Se hizo un moño = She put her hair in a bun
• Se recogió el pelo en un moño = She gathered/put her hair up into a bun

The second version is a bit more explicit because it mentions el pelo directly.

Not naturally for this meaning.

If you say mi hermana hizo un moño, it sounds more like she made a bow/knot/object, not that she styled her own hair.

For a hairstyle, Spanish normally wants:

• Mi hermana se hizo un moño
• Mi hermana se recogió el pelo en un moño

So the se is important here.

The ñ is pronounced like the ny sound in English canyon.

So moño sounds roughly like:

• MO-nyo

Not exactly English pronunciation, but that is a useful approximation.

It breaks down like this:

• Mi hermana = subject
• se hizo = reflexive verb in the preterite
• un moño rápido = direct object phrase
• antes de salir de casa = time expression

So the sentence structure is:

Subject + verb + object + time phrase

That is a very normal Spanish word order.