Use a dictionary or the thesaurus.
He does sit-ups every day and his abs are as hard as a rock.If you leave the play dough out all night, it will be as hard as a rock by tomorrow.
It is really hard to find a sentence with the word hard in it.
2 ways the work was hard and tiring cheddar is a hard cheese
Hard is an adverb in the sentence. The word hard does not require 'ly' to make it an adverb