Modernism affected the literature of the Harlem Renaissance in that it gave somewhat of a new freedom to writers of this place and time. Modernism promoted freedom of expression by way of more experimentation, which included "stream of consciousness" writing. Modernism was all about change and this applied to many areas of life, and by extension it filtered into the arts and the writing discipline more specifically.
Therefore, writers from the early 20th century to approximately the time-period around and just after World War II heavily engaged in Modernism. Its effects have affected the works of writers to this day. Its effects certainly affected the output of writers and others who were associated with Harlem in New York City.
Writers explored their black heritage and manifested their explorations and knowledge into their writings. The Harlem Renaissance developed even more through the aforementioned experimentation and the desire to push the envelope so to speak concerning artistic expression - to say new things in new and innovative ways.