Remember that you need to use an adverb (-ly) when describing how an action is done or when modifying an adjective (where the verb “to be” is implied).
Otherwise, as in the case above, you’ve left the adverb enormously as an adjective modifying a noun somewhere rather than a verb, and you’re liable to find that you’ve changed the meaning. Calling the person enormous probably wasn’t the intention. A couple of real-life examples:
- marketing wishes of extreme fast-growing organizations => extremely
(I doubt the company was extreme.)
- these items were coded reliable => reliably
(A bit wonky, but I’d probably read it as meaning that the items were encoded to make them reliable, not that the encoding process was done without errors.)
It becomes less clear if you have a multi-word phrase that you are using as an adverb, such as “off the charts” instead of “enormous” above. There’s nowhere to put the -ly, so they tend to get hyphenated and left as-is, in American English in particular. Although many native writers would be uncomfortable with the rather informal result and choose to rephrase it.
Prevalence: moderate. Most Dutch authors are aware of this. But it sneaks in at times and is worth noting because it can so easily change the meaning.
Frequency: low. Adverb usage in formal writing is fairly low anyway.
Native: no. Definitely not for single words, at any rate.