The problem once again has root in failing to distinguish between written English and spoken English. As I’ve posted previously, these two forms of communication have the same language but different conventions. Spoken English has more flexibility because its conventions are not as rigidly defined as those of written English. Thus, failure to distinguish between the two can lead to less effective writing.
Bad and badly provide a great example of this condition. Both words carry the same denotation of not good, but they are different parts of speech. Bad is an adjective, whereas badly is an adverb. This difference matters considerably because adjectives modify nouns whereas adverbs modify verbs, adjectives, and other adverbs.
For instance, consider the example sentence in the cropped graphic above. While appropriate in spoken English within some circles, the use of bad in this sentence is just bad in written English. That’s because the writer used an adjective to modify a verb (corroded), thereby introducing a structural inconsistency. Guess what? Structural inconsistencies don’t make effective technical writing.
The writer can improve the sentence by using the correct part of speech.
Once exposed, the iron core corroded thoroughly.
Once exposed, the iron core corroded unabashedly.