not hard to, easy to, is obvious that

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

difficult

(archaic) difficult

difficult or hard to do

difficult or hard to do (emphasis on the disagreeability of performing the action)

negative form of 難い / (literary) easy to

(informal) no problem

unthinkable, out of the question

practical question (problem)