Difficult
Part of speech: adjective
Definition: hard, not easy, requiring much effort
Example sentence: Knowledge comes, but wisdom lingers. It may not be difficult to store up in the mind a vast quantity of facts within a comparatively short time, but the ability to form judgments requires the severe discipline of hard work and the tempering heat of experience and maturity.
Intractable
Part of speech: adjective
Definition: Not tractable or to be drawn or guided by persuasion; not easily governed, managed, or directed; uncontrollable; incurable; violent; stubborn; obstinate.