Sometimes when the body part is modified by an adjective the possessive is used, it mostly happens in poetry I believe, but to be honest, this is not a good example, as it's something a native speaker would not say in actual speech, and the poetic version would be more like "Ella se cepilla su largo cabello". As it stands right now, it sounds like she's got another set of hair apart from the long one.

I'm pretty sure it's an error. The reflexive indicates it's her hair so there's no need to say it.

It would be nice if someone who knows for sure why "su" is used here instead of "el," would explain it. Please.

07/14/18. Very good educated guess, lv2jft8s. It is the adjective "largo" which at least permits (if not necessitates) the use of the possessive adjective "su" in this exercise example.

"When the thing possessed is emphasized or particularized by context, or by an adjective or some other words, or whenever ambiguity must be avoided, the possessive adjective usually reappears: [ ] Vi sus ojos grandes, fatigados, sonrientes y como lacrimosos. I saw her eyes, big, tired, smiling, and seemingly tearful." Butt, John and Carmen Benjamin, A New Reference Grammar of Modern Spanish, § 8.4.3 (Definite article instead of possessives), p. 96 (4th ed. 2004).

But note that the above contrasts with another example from www.thoughtco.com/body-parts-vocabulary-3079570, where a definite article is used with a body part modified by an adjective: "Tengo el pelo ❤❤❤❤❤. (I have black hair.)"

Apparently, choosing the use of a 'possessive adjective' over a 'definite article' with a body part modified by an adjective is more optional than mandatory in Spanish, with the 'possessive adjective' being chosen when the intent is to place greater emphasis on the modified 'body part' noun.

I had the same problem. Accepted when I added "se": "Ella se esta cepillando su cabello largo."

i think i got it; cepillarse is the infinitive form so for example i could say "he wants to brush his hair" - "el quiere cepillarse el cabello" as opposed to he's brushing his hair "el se cepilla el cabello"

Quick and simple; first verb needs a prefix in front, second verb in a row will need it to be added behind. "Tiene que" is also considered a verb so the next verb coming cannot have a prefix.

El quiere cepillarse = two verbs - El se cepilla = one verb

This didn't turn out simple at all. :)

I got confused by this as well because i didn't notice that it was one of those reflexive verbs... meaning "i brush my own hair" vs. "I brush hair". The former needs the extra me/te/se/etc. before the conjugation. At least that's how i made sense of it later.

El is correct also. Accepted 2/14/19

'el cabello' is not wrong

Duo accepts either su cabello or el cabello.

Yes, why is it cabello? After all, you go to a peloteria to get your hair cut, not a cabelloteria.

I am assuming that "cabello" is an American usage. I was taught "pelo" by a native Spanish speaker.