【Hard】 ,  【difficult】 ,  【arduous】  are comparable when applied to tasks for mind or body to mean demanding great toil or effort in reaching the appointed or the desired end.

【Hard】  is the simpler, blunter, and more general term; it implies the opposite of all that is implied by easy, but usually suggests nothing more specific.

【Difficult】 commonly implies the presence of obstacles to be surmounted or of complications to be removed; it therefore suggests the necessity for skill, ingenuity, sagacity, or courage.

【Difficult】  is more widely applicable than  【hard】 , because it often means specifically 【hard】 to understand because abstruse, intricate, or abstract, or 【hard】 to deal with because thorny, knotty, cumbersome, delicate, or exacting.

【Arduous】  stresses the need of laborious effort, of perseverance, and persistent exertion; thus, one may find a task  【difficult】 , but not  【arduous】 , because one has no sense of being kept at it against one’s inclination; an ascent of a mountain may be  【arduous】 , but not especially  【difficult】 .