is 'acabado' acceptable instead of 'terminado'?
I certainly believe so.
By itself, "acabar" means "to finish" or "to end".
"He acabado este capítulo." Accepted today 9/5/2020.
I submitted 'He terminado este capitulo,' and was marked incorrect. It it really necessary to add 'yo?'
No necessary, Duo just doesn’t have your answer in their database.
I think 'acabar' (usually used with 'de') means 'just finished' as in 'he acabado de escribir un mensaje a duo'
Why not use: esto capitulo
Because capïtulo is a masculine noun and esto is a neutral pronoun. The masculine noun calls for the use of a masculine pronoun, and therefore, the pronoun has to be este.
Why is yo there?
He cumplido este capitulo. What is wrong with this.
why is it not "terminado de"?
"De" in only required to connect with a following infinitive.
Terminar + Noun
Terminar de + Infinitive