"I open the window."
Well, it might be more logical that way but knowing Duolingo and its weird sentences, I think they are focusing here on the present tense, so they put on "open" (أفتح) instead
in Arabic there really no "continuous" tense. Just a simple tense. Maybe to inflect the continuous status, one can add, for example: إنني (Innaní) (i am indeed, verily I am, emphatic "I"); Or simply add الآن (al-án) which means (now) to the end of the sentence.