what does -il- do?

 I was thinking again (no no, don't run YET)....and became curious as to
what matter of modifier (if that's the right word) -il- is.

 For example...
"The wood is heavy."
"The forest is heavily wooded."

 one does not (that I know of) say "the forest is heavy-wooded" or "the wood
is heavily"...so I thought to ask: what is the -il- that so affects some
words?  *curious*

 thanks.

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