"Hard" has two meanings but "difficult" only has one. Hard can mean a physical property of a substance, for example, "The concrete is hard," or "wood is hard." Hard can also be synonymous with "difficult." "The test was hard," or "it was hard work." Difficult is just used to describe tasks or actions. "The test was difficult," or "it was difficult work," or "the French horn is a difficult instrument," mean the EXACT same thing as "the test was hard, "it was hard work," or "the French horn is a hard instrument." Difficult can NOT be used to describe physical things. "The concrete is difficult," or "the wood is difficult," are incorrect and meaningless statements. Hope I helped!