"He said that he had opened the window."
Translation:Él dijo que él había abierto la ventana.
in another example in this same round, duolingo accepts the lack of subject in the last participle. "ella habia demonstrado que puede escribir bien." why is is required here? the sentence structure is a little different, but it seems like the same basic concept in both sentences. Two separate ideas with the same subject being combined in one sentence.