The following sentence bothers me:
As
a result, most of what native speakers “know” about their language is burned
deep into the circuitry of their brains.
What I cannot seem to come to grips with is the word "deep." It is, to me, obviously an adverb, so it should be "deeply." However, when I make that substitution, my internal grammar checker informs me that that's wrong. What's going on here?