Q. nobody is learning
In nominative case, use "Keiner" for masculine nouns like ""
I thought in nominative it was either kein or keine,
@ Grafiker: if I understand your question rightly, you asked why it is "keiner" instead of "kein". The reason is, that "keiner" is used as a pronoun. "kein" would be used in combination with a noun, like "kein Mann". The pronoun gets the endings of the article. (der>>keiner, die>> keine).
If I understand correctly, you might be thinking of the accusative case - for example, "Ich habe kein Pferd" or "Ich habe keine Blumen." However, when "kein" is going to be the subject of a sentence (nominative), it must have the "-er."
Since the noun has to be inflected in some way the pattern to inflect it is the masculine way, and in German that is the case with many words. When we're just talking about a person in general we use "keiner" and inflect it like it is a maskuline noun. Example: "Keiner lernt" , "Wir sehen keineN" , "Er hat mit keineM gelernt". It's the same thing with "einer" (roughly translated: someone), "who" = wer. It has an "er" at the end. Grammatically it is masculine even though this "who"/"einer" could be a woman. Example: "Wer lernt?" , "WeN sehen wir?" , "Mit weM hat er gelernt?". It's the same thing with words like "jeder" (everyone) "niemand" (no one).