It doesn't and you're right.

Well yes, sometimes Duolingo introduces elliptical sentences or phrases.

Well, the pattern XaXiX is mostly used to indicate the capability of being acted upon, for example קָרִיא legible, כָּבִיס washable, whereas added -י mostly derives adjectives from nouns, for example אִישִׁי personal, יַמִּי marine. But like every organically grown language, there is a proliferation of word forms using these word pattern more freely. If you yearn for a more systematic word formation, maybe Esperanto would give you greater pleasure. PS. מָהִיר was originally not the antonym of slow, meaning biblically skilled, which is the meaning of this root in other semitic languages.