Language is a mutable thing. I remember being taught that as a kid. I know it’s true. In the past 20 years, I have watched with interest as adverbs have disappeared. When I started teaching English, kids had no trouble identifying adverbs. They were easy to spot, since most of them ended in -ly. We’d find the verb of the sentence and ask how? He walks. How? Joyfully! Quickly! Sadly! Carefully! Morosely!
Now kids struggle to find the adverb because the easy identifier has vanished. When people speak these days, they often use adjectives in the place of adverbs. How does he walk? Quick. Sad. Morose. Careful. Joyful. It is confusing for kids to learn the ‘correct’ option versus the common usage. It does not help that the government does it, too.
Ever seen this sign? DRIVE SLOW!
ARGGGGGGG!!!!!!!!
Every time I see it I re-state it correctly: DRIVE SLOWLY!!!
Seriously, how expensive would it have been to add those two letters and make it a correct sentence? Wasn’t there anyone at the Public Works Yard who knew correct grammar usage?
>>sigh<<
We can’t fight the tide. Language changes. It is the essential nature of living languages that they grow and alter. It’s just really hard on the English teachers.
(and don’t get me started on ‘their’ being used as a gender neutral singular pronoun instead of a plural!)
Shawn L. Bird 
Slow, like fast, is an adverb as well as an adjective. It has been an adverb for centuries. Shakespeare used it. “Drive slow” is less formal than “Drive slowly”, but grammatically it’s fine.
I was stunned by this comment. I did an exhaustive investigation. Indeed, within the century after ‘slowly’ was first introduced, people were apparently starting to use ‘slow’ as an adverb form. I am still quite discombobulated, because it sounds wrong, wrong, wrong. My only consolation is that language evolves, and whether we like it or not, the grammar does not stay the same. You can drive slow if you want, but I’ll be driving slowly, and enjoying my slow drive.
Thanks for responding, Shawn, and sorry to have stunned you! As you say, most adverbs end in –ly, but there are quite a few familiar ones that don’t, such as quite, soon, almost, and very. They are most reliably distinguished from adjectives not by appearance but by function. Weird Al got it wrong. There’s a helpful overview of the commentary on slow vs. slowly in the Merriam-Webster Dictionary of English usage.
I will be driving slowly too, but there may be occasions when “drive slow” seems more apt. As a writer, I like to keep my options open.
I am well versed in intensifying adverbs like really, quite, almost and very. As well as those traditionally formed adverbs which answer “when, where and how.” I am stunned by the fact that ‘slow’ which should be an adjective only, to my mind, can also be an adverb, when there is a perfectly clear adverb form already (‘slowly’) Why do we have one that is only an adverb (slowly) and one that is both an adjective and an adverb? (slow). That is redundant and silly. If we had an equivalent to the Academie Francaise such a thing could not have happened, but in English we accept the fluidity of language, and people who are too lazy to add their -ly get to change language. ;-P C’est la vie. It’s still shocking. In fact, it was that Merriam Webster article (and a similar one in OED) that convinced me that you were actually right, because I absolutely did not believe you were when I first read your comment!
Something new to learn every day. I am the grammar expert at most schools I work in. It is rare that I am wrong! lol Perhaps that is the greater shock? lol A little humility is good for the soul.
Languages are full of redundancy; it’s embedded in their structure at multiple levels, and in many ways is a feature as opposed to a bug. Words have always taken on multiple functions (hence this absurd but grammatically valid sentence). It’s one of the main ways languages grow and evolve.
Likes and dislikes don’t enter into it so much. Give me variety and silliness over an official academy and its “One Right Way” strictures any day!