elusive, illusive

Elusive or elusive means mentally difficult to grasp, eluding a clear perception or understanding--
The concept of democratic government may remain elusive, unless the students are offered real-life illustrations and examples of how our government works.
Algebra seems elusive, until practice makes the idea of a variable seem logical.

Elusive may also mean difficult to catch, or skillfully evasive--
"Across my dreams, with nets of wonder, I chase the bright elusive butterfly of love..."
He got a hook into the elusive old trout, but it got away.

Illusive or illusive means decptive or unreal--
They spent years searching for the illusive fountain of youth.
The illusive philosopher's stone was said to be used to turn lead into gold.

Now that you know that, you can say--
"He is trying to explain an elusive concept that many people believe to be illusive."

Note--"illusory" is more commonly used than "illusive"; it has the same meaning--
Your fears are illusory; the noise was only the wind.
Note # 2--"elusory" is less commonly used than "elusive"; it has the same meaning--
These concepts won't be elusory if you work on a lesson that is age-appropriate.