一、hardness   hardship

1 hard做形容词“坚硬的”，对应名词为hardness，意味硬度，坚硬;
2 hard做形容词辛苦的、努力的”，对应名词为hardship，指经历的辛苦，困苦，苦难。

Hardness is a quality - the quality of being hard: “He lists the relative hardness of different materials, from talc to diamond.”

Hardship is the fact of something being hard to bear: 'Throughout his epic voyage he had to put up with many hardships, from frostbite to near-starvation.''

二、difficultness   difficulty 

difficultness　困境；困难的状态　　difficulty　困难；难度

Ｔhey agreed on the difficulty of the climb.

This is an article on the difficultness of reforming in China.