This question requires distinguishing the meaning of the words 'Hard' and 'Hardly'.
'Hard' can function as both an adjective and an adverb.
'Hardly' is an adverb that signifies a very small or insufficient amount or degree.
Let's evaluate the options based on these definitions:
Therefore, the most accurate description matching the common usage and grammatical function of both words is that 'Hard' means difficult to a great degree and 'Hardly' means scarcely.