Me cepillo el pelo antes de salir.

Because cepillarse is commonly used as a reflexive/pronominal verb when you brush your own hair.

• Me cepillo el pelo = I brush my hair
• Cepillo el pelo de mi hija = I brush my daughter’s hair

In Spanish, actions you do to your own body often use a reflexive pronoun:

• me lavo las manos = I wash my hands
• me cepillo los dientes = I brush my teeth
• me peino = I comb my hair

So me shows that the action is connected to the speaker.

With body parts, Spanish usually prefers definite articles (el, la, los, las) instead of possessives like mi, tu, su, especially when the owner is already clear from the reflexive pronoun.

So:

• Me cepillo el pelo literally looks like I brush myself the hair
• But it naturally means I brush my hair

This is very common in Spanish:

• Me lavo las manos = I wash my hands
• Se rompió la pierna = He/She broke his/her leg

Using mi pelo is possible in some contexts, but here el pelo sounds more natural.

No. It depends on what or whose hair you are brushing.

• Me cepillo el pelo = I brush my own hair
• Cepillo el pelo del perro = I brush the dog’s hair
• Le cepillo el pelo a mi hija = I brush my daughter’s hair

So cepillarse is used when the action is done to oneself, but plain cepillar is used for other objects or other people.

Because Spanish usually drops subject pronouns when the verb ending already shows who the subject is.

• cepillo already means I brush
• So (yo) me cepillo el pelo and me cepillo el pelo both mean the same thing

Spanish includes yo only when it is needed for:

• emphasis
• contrast
• clarity

For example:

• Yo me cepillo el pelo antes de salir, pero mi hermano no.

Because after antes, Spanish normally uses de before an infinitive.

So:

• antes de salir = before leaving / before going out

This pattern is very common:

• antes de comer = before eating
• antes de dormir = before sleeping
• antes de estudiar = before studying

So antes de + infinitive is the standard structure.

Because after a preposition like de, Spanish uses the infinitive when the subject is the same as the subject of the main verb.

In this sentence, the same person does both actions:

• me cepillo = I brush
• salir = I leave / go out

So Spanish uses:

• antes de salir

If the subject changes, Spanish usually uses antes de que + subjunctive:

• Me cepillo el pelo antes de que llegue Ana.
  = I brush my hair before Ana arrives.

It can mean either, depending on context.

• antes de salir can mean before leaving
• It can also mean before going out

In everyday speech, both are possible. The exact meaning comes from the situation:

• leaving the house
• going out socially
• heading somewhere

So the Spanish phrase is flexible, just like English can be.

Yes. Both can refer to hair, but pelo is usually the more common everyday word in much of Latin America.

• Me cepillo el pelo = very natural, everyday
• Me cepillo el cabello = also correct, sometimes a bit more formal or careful

Very general tendency:

• pelo = more common in daily speech
• cabello = often sounds a little more formal or descriptive

Both are understood.

Usually it sounds like a habit or a normal routine:

• Me cepillo el pelo antes de salir.
  = I brush my hair before leaving / before I go out.

Spanish present tense often covers things like:

• habitual actions
• routines
• general truths
• sometimes near-future actions

Without more context, this sentence most naturally sounds like a routine.

Yes. Spanish allows some flexibility.

For example:

• Me cepillo el pelo antes de salir.
• Antes de salir, me cepillo el pelo.

Both are correct. The second version puts more focus on before leaving.

The original order is very natural, but moving the time phrase to the front is also common.

Because pelo often works like a collective singular when talking about someone’s hair in general.

So:

• el pelo = hair, as a whole
• los pelos = hairs, individual hairs, or sometimes body hair in a more literal sense

That is why Spanish normally says:

• me lavo el pelo
• me corto el pelo
• me cepillo el pelo

Using los pelos here would sound unusual for normal everyday grooming.

Yes, but it is not exactly the same.

• Me cepillo el pelo = I brush my hair
• Me peino = I comb/style my hair

Sometimes they overlap in everyday meaning, but cepillar specifically involves a brush, while peinar is broader and can mean arranging or combing the hair in general.

So if you want the idea of specifically using a brush, me cepillo el pelo is more exact.