Is adverbial ‘deep’ used wrong?

The word deep runs deep in English history. In Old English it served a range of grammatical functions, much like today. It was used as a noun meaning deepness or the deep part of the sea (or other body of water). It was a verb meaning to make deep (= deepen), a usage now obsolete. It worked as an adjective in the sense still familiar to us, and as an adverb equivalent to deeply which could refer to either physical or figurative depth; this too mirrors current use: She drove deep into the countryside and went deep into the investigation.

Yet despite its long-established status in the language, adverbial deep is sometimes viewed with suspicion, as though there’s something not quite right about it. Take my opening line: ‘The word deep runs deep in English history.’ Does it read OK to you, or would you favour the phrase runs deeply?

There is a common misconception that all adverbs end in –ly. Because we are taught as children that many of them do, we may come to imagine that it’s a rule, not the broad (but limited) pattern it really is. We may be unaware of the precise nature or standing of flat adverbs. Or we may have inherited prescriptive grammarians’ mistaken idea that flat adverbs are adjectives being used wrong. Or wrongly, as they would insist. This position is remarkably stubborn given how much we have learned about language since the 18th century.

Confusion is especially likely when the two words are synonyms. No one would argue that They arrived late should be They arrived lately, because late and lately mean different things. But people often claim that Drive slow should be Drive slowly, even though both are fully grammatical. Some insist that I’m good should be I’m well, though the meanings here differ a little. On a recent newspaper article about commonly misused words – such articles usually mislead, by the way – a reader commented that the phrase used wrong should be used wrongly because ‘an adverb is needed’.

These claims are typical of amateur language policing: adoption of a half-remembered rule that was dubious to begin with, leading to dogmatic repetition of the rule without checking it. If the critic had bothered to look up slow, good, or wrong in a modern dictionary, they would have found that they’re all in acceptable use as adverbs. Drive slow, I’m good, and used wrong are fine, and you don’t have to dig deep to find out.