What is the difference between difficult and hard?
Feel free to just provide example sentences.

"Hard" has two meanings but "difficult" only has one. Hard can be used to describe how something feels, for example, "The concrete is hard," or "wood is hard." Hard is a synonym of the word "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," do not make sense are are used incorrectly.