[Grammar] the Use of "Imperceptible"?

[mcpig]

This is some sentences from my essay...

"In the speech, he introduces to the graduates the term “Inner Rings”, and explains how the graduates should handle them. The central idea is clear presented to his audience: In every social group there are one or more inner rings which are unavoidable. It is crucial for everyone to be aware of the desires for getting into the rings because these desires are recurring and harmful in an unpredictable way. "

The red word was "imperceptible" before but my instructor said it is a wrong word.
I can use unpredictable but I think it does not clearly show that I mean "a way that is hard to notice"..what word should I use?

P.S. is there a place in this forum where I can post my paragraph or essay for checking whether I use some words and sentence structures appropriately? Wording is always my problem.
thank you.

[jlinger]

If you agree that "unpredictable" is also correct, but you want to say that sometimes it's hard to see the result, then you might simply say that: to wit, "unpredictable and sometimes imperceptible ways." I think the reason that your original statement (flat out imperceptible) was revised, is that it is hard to imagine things that are harmful being imperceptible. If you can't perceive them, then their damage is difficult to acknowledge.