What is the difference between modernism and modenaty?my Q about poetry subject

I agree-- I'll add that besides the particular interests jk180 identified for modernist artists and writers, modernism is characterized by a commitment to stretching the potentialities of form (language, image, etc.).  Thus in modernism you'll find experimental styles and techniques like stream-of-consciousness, free verse, montage, etc.

While modernism describes cultural production, modernity is a term for a historical period.  The reason I think the terms feel so hard to define is that modernity is demarcated by a series of technological advances that deeply influence and determined the concerns of modernism.  (This as opposed to more rigidly demarcated periods, such as the Victorian Age, which coincides with the reign of Queen Victoria of England; note that the Victorian Age is coincident with modernity.)  Modernity is the period after the medieval period for some, since that's the technical transition from agrarian production to a more urban-centered, market economy.  Some think of modernity as beginning with the advent of (mechanical) industry, the Industrial Revolution (1830s).  The end date is likewise indefinite, but it is important to note that we are no longer in "modernity," according to the perspective I personally adhere to; we're in "postmodernity."  Generally the transition from modernity to postmodernity is located in the shift from national markets to global capital, and is also associated with a sense that the modernist (Enlightenment) faith in reason and progress has not delivered.

Another thought: modernity is often quite different in its commitments from modernist artistic production; especially in the case of the avant-garde, modernism is often a direct rejection of modernity.  Not always.  For instance, the modernist movement Surrealism rejects the modern emphasis on the empirical, the rational, etc., by emphasizing subconscious or aleatory sources for art.  On the other hand, Russian constructivism is committed to a practical use of art toward social progress (a modern ideal), and American precisionism finds beauty in the geometrical shapes of factories.