"Next time, pay attention during the lesson."
Translation:В следующий раз будь внимательна во время урока.
I often feel at unease when Duolingo rejects a translation merely because of the word order (I had будь внимателен в следующий раз во время урока). Although I am willing to accept that, in the present case, DL´s version is probably preferable, I wonder whether the alternative translation is really "wrong" or at least too clumsy.
Can someone explain the declension внимательна, внимательний, внимательнен? I don't understand what exactly we are declining here. (You) pay attention during the lesson. The subject (You) is assumed. How do we know if "you" is masculine, feminine, plural or singular here?