What is an inflected noun? Duolingo says that predicate adjectives are not inflected. What does that mean?
A predicate adjective comes after the noun to describe the noun. For example: 'The apple is green.'
In German, when an adjective comes after a noun, it is not inflected. (I.e. you don't change the ending of the word depending on case/gender/etc.)
Der Apfel ist grün = the apple is green.
However, if the adjective came before the noun, it would be inflected, and its ending would change depending on a number of factors.
Der grüne Apfel = the green apple.
As to your question 'What is an inflected noun' - A noun is inflected (its ending is changed) depending on gender/case/etc.