I still remember the awful woman I met at a reception during an English Speaking Union meeting on George Street, Edinburgh, in 2008 (I mentioned her here once before). She told me loudly and confidently, as if playing Lady Bracknell on stage, that English was rapidly degrading; for example, “The Americans have no adverbs. Absolutely none. They’ve just got rid of them.”
I wanted to explain about my American citizenship and quarter-century of living and teaching linguistics in California, and the many adverbs I had encountered; and about my experience co-authoring The Cambridge Grammar of the English Language (one of the chapters for which I did the first draft was Chapter 6, “Adjectives and Adverbs,” so I do have some relevant specialist knowledge here). But I could not stop the flow; she was in output mode, a conversational steamroller. She rolled right over my attempted protestations. I snatched an excuse (“Just a moment, I need to have a word with someone over there”) and abandoned the one-way unconversation.
The saddest thing is that I know exactly what she had in mind when she made her apparently deranged remark. The very same ideas came up in a recent email from an old school friend of mine, back in touch after several decades, who said (regarding British English rather than American):
Increasingly people who to my mind should know better are following this recent trend of using an adjective in place of an adverb. One could argue that this doesn’t matter as the meaning is still clear. But then why bother with grammatical rules at all? English is evolving all the time but not necessarily for the best.
Notice first that many English adjectives, on both sides of the Atlantic, have corresponding adverbs with no extra -ly suffix: hit it hard, long forgotten, finished it earlier, arrived late, and plenty of others. What my friend has correctly noticed is that in informal speech other short adjectives often behave similarly, so we find adverbial uses of words like quick (Get down here quick!), bright (The sun was shining bright), etc. Among American speakers, this tendency is stronger — which is doubtless what the steamroller lady had noticed.
But people unused to grammatical analysis miss relevant details. One is that these unsuffixed adverbs have a different syntax from the suffixed ones. Consider a familiar informal-style expression like :
 I’ve tried it, and it works great.
The dragon of George Street would say that this is just typical degraded American English: Having slaughtered all their adverbs, they cluelessly use adjectives instead. But wait a minute: Adding the adverb-forming suffix -ly here doesn’t work!
 *I’ve tried it, and it works greatly
If  is a sentence at all (I’m inclined to say it isn’t, hence the asterisk), it certainly doesn’t have the meaning of . So  cannot be understood as the result of an illiterate failure to add the adverb-forming -ly. It’s more subtle than that.
The adverb greatly does exist, with a meaning something like “to a large degree,” and interestingly, it doesn’t occur in exactly the same places as the unsuffixed adverb great. Like most adverbs of manner and degree, greatly can be placed either after the verb or before:
 I’ve heard they improved the bus service greatly.
 I’ve heard they greatly improved the bus service.
But that’s not true of unsuffixed great. Compare  with :
 *I’ve tried it, and it great works.
This is not an idiosyncrasy of great. The unsuffixed adverbs — great, quick, bright, etc. — quite generally have a different syntax from the standard suffixed ones. Compare quickly and quick:
 You should eat it up quickly before someone else gets it.
 You should quickly eat it up before someone else gets it.
 You should eat it up quick before someone else gets it.
 *You should quick eat it up before someone else gets it.
Far from having no grammar, or only a degraded and chaotic grammar, in the relevant style of colloquial American (or the British dialects that use adverbs like quick or great) there is an extra rule: The extra unsuffixed adverbs that look like their related adjectives differ from the suffixed ones in that they are not permitted to precede the verb they modify.
Such facts seem fascinating to me. It’s sad that so many people are blind to them. Though perfectly capable of distinguishing total extinction from decline in numbers or shift of range when wildlife ecology is under discussion, they lose their critical faculties when confronted with language. Perceiving none of the subtleties, they start voicing paranoid fantasies about adverbs disappearing. Ecology may not be as well understood by the general public as it should be, but it’s doing better than grammar.