According to Dictionary.com
difficult [dif-i-kuhlt, -kuh lt]
- not easily or readily done; requiring much labor, skill, or planning to be performed successfully; hard
a difficult job.
- hard to understand or solve
a difficult problem.
3.hard to deal with or get on with
a difficult pupil.
adjective, harder, hardest.
1.not soft; solid and firm to the touch; unyielding to pressure and impenetrable or almost impenetrable.
2.firmly formed; tight
a hard knot.
3.difficult to do or accomplish; fatiguing; troublesome
a hard task.
etc. (many more)
In what situations is there a preference over the other? What are the rules for that?