It isn't. "The teacher learns" is a correct translation, too, and Duolingo should have accepted it.
Ah, okay. I didn't do the exercises so I assumed that the task was to translate the sentence.
Previously it said teacher was a feminine noun now it says its masculine. How so?
"The teacher" can be translated as either "der Lehrer" or "die Lehrerin".
der Lehrer = the (male) teacher, masculine noun
die Lehrerin = the female teacher, feminine noun