In a not-so-long-ago ad, Apple asked us all to “think different.” Even longer ago, Elvis Presley asked us to “love me tender. And when I was a wee bairn, my mum used to tuck me in at bedtime with the phrase, “Night night, sleep tight, don’t let the bed bugs bite.”
I wasn’t a particularly precocious or bright toddler, so my response to mum was simply to smile and stick my head under the covers to check for insects, rather than, “But mum, surely it should be sleep tightly because you’re using the word as an adverb and therefore the correct formation of the word is to take the adjective as the base and use the –ly ending as an adverbial morpheme?” I suppose if I has said that I’d have been called a “clever clogs”  and told to “just go to sleep.”
Adverbs, by definition, are used to describe verbs, adjectives, and other adverbs. With Elvis, if the question to him was “How do you want me to love you,” he should reply “tenderly”; with Apple, if the question was “How would you like us to think,” the reply would be “differently”; and with mum, she should be telling me to sleep “tightly.” We might also find we’re “talking loudly,” “laughing heartily,” “arguing vehemently,” “working quickly,” and “complaining bitterly” whenever the occasion demands it.
So why isn’t Apple thinking differently, Elvis being loved tenderly, and I sleeping tightly? Well, it’s all to do with something called flat adverbs and the appeal of the –ly ending.
The commonest way to create an adverb is to take an adjective and add an –ly to the end of it. You have a “hungry cat” and a “thirsty dog” but the former will “eat hungrily” and the latter “drink thirstily.” Similarly your “perfect day” should “end perfectly” and a “generous patron” will always “give generously.” It’s regularity like this that should make the lives of teachers of English easier, and the possibility for artificial intelligence more likely. Alas, consistency and continuity seem to be in short supply when it comes to language. In fact, just when you think you’ve got it all worked out, the lexical world starts to wobble on its axis and, like tectonic plates on a bed of molten rocks, words slide around and rearrange themselves in all sorts of non-standard ways.
Flat adverbs are an example of these slippery words that want to have it both ways – adjective and adverb. It’s like Bruce Jenner wanting not to become just Caitlyn but both Bruce and Caitlyn at the same time! They skip and jump around like frogs on a hot plate, not pausing long enough for anyone to get a grip on which is right or wrong – or perhaps more accurately which is better at any particular time.
One situation where you can take a stab at which to choose in when you’re writing songs or poems and meter is important. When mum told me “Night night, sleep tight,” she was simply adhering to the underlying stress pattern of the phrase, along with the rhyme for night and tight. The form “Night night, sleep tightly” would be judged grammatically correct but poetically wrong. Similarly when Johnny Cash sang about how “the sun shines bright on my old Kentucky home,” using brightly would have buggered up the timing , forcing the Man in Black to slip in an extra syllable that really doesn’t want to be there. And try singing “Love me tender-ly, love me do…” to get a feel for why Elvis flattened his adverb.
Our confusion over flat adverbs is comes primarily from those that are identical to an adjective. If you consider the pair fast and slow, the former presents less of a problem because it doesn’t have an –ly form. I can “run fast” (adverb) or drive a “fast car” (adjective) and not worry about whether it’s an adverb or an adjective because there simply is no *fastly. However, although I can drive a “slow car” (adjective) it’s less obvious whether to “drive slow” or “drive slowly.”
As you might suspect, the bastard nature of English also plays a part in spreading confusion . Way back when Old English was the current flavor of the language, changing an adjective into an adverb was done by the addition of a final -e; fairly simple, eh? So if we had the word glaed (OE for our modern glad) then you could add an e to make glaede meaning gladly. So far so good.
If you wanted to turn a noun into an adjective, you could add the ending -lic; again, not to tricky. The word craeft (meaning skill) became craeftlic, an adjective meaning skilful. So guess what you did to say skillfully? Yup, you added the e-ending to get craeftlice.
This meant we had some adverbs ending in a very weak-sounding –e and others with a more pronounced-sounding –lice. Gradually over the years, the weak –e disappeared and the stronger –lice became the slightly weaker –ly. Equally, those adjective ending in –lic also wore down to take on the sound of –ly. By the 14th century, we had adjectives and adverbs ending in –ly but this ending became the more commonly used to mark adverbs. Folks then started adding it willy-nilly to adjectives and this is pretty much how we do things in Modern English.
It’s not surprising that folks have some trouble working out whether adverbs should have an ly at the end or not, and those fossilized flat adverbs don’t make it any easier. Strang (1970)  expressed a sentiment that is as true today as it was in the 20th century:
…the sense of unease about adverbs homophonous with an adjective […] has been felt at all periods, and there has been a steady progress from plain to –ly forms (p.273).
Apart from my earlier suggestion that you can use poetic meter to decide which word to use, another guideline you might want to consider is that flat adverbs are more likely to sound right in short, imperatives. So “sleep tight” and “drive slow” are fair enough. As is “think different.” As always, if you’re unsure, use a dictionary or better still an online corpus. But don’t get too wound up about whether to use an ly form of not; if it’s taken a thousand years to get to this point where no-one is sure, you’re not going to find the definitive answer from reading this one blog post!
 I’m something of a fan of the UK cartoon series Peppa Pig, and in an upcoming post I’ll explain in some detail precisely why but for now, just take this as a snippet of information that gives you a peek into what makes me tick. In several episodes, the phrase “clever clogs” is used, and although I had to explain this to my American family, folks over in the UK have no difficulty with it. And why not, seeing as it appears to have been around since 1866 at least! Joseph Wright’s 1898 English Dialect Dictionary also includes the phrases “clever-breeches,” “clever-clumsy,” “clever-dick,” “clever-head,” and “clever-shanks.”
 When I was a kid in the 1960s, the word bugger was a swear word that would get me a clip round the ear for using. In the hierarchy of swear words, bugger was about as profane as bloody, with bloody hell being a tad more shocking. In the more liberal 21st century, bugger and bloody are now little more than quaint Britishisms, especially to the American ear because they never crossed the Atlantic as curse words. It’s a little known fact – but allow the Dudes to enlighten you! – that the word bugger comes from the Latin Bulgarus, which means Bulgarian, and was used to refer to a group of 11th century heretics who came from Bulgaria. As often happens when people talk about any group with which they disagree, the orthodoxy ascribed certain “practices” to the Buggers, one of which was sodomy. By the 16th century, the word was being used to describe anyone who committed the crime of buggery (engaging in sodomy), and by the 19th century it was being used as a general term of abuse or insult. By the end of the 20th century, it had become less profane and could also be used in a more affectionate”blokish” way, such as “He’s really quite a decent bugger when all’s said and done.”
 An interesting article on the development of the ly-ending in English and its parallels in other languages is:
Hummel, M. (2014). The adjective-adverb interface in Romance and English. In P. Sleeman, F. V. d. Velde & H. Perridon (Eds.), Adjectives in Germanic and Romance (pp. 35-72). Amsterdam and Philadelphia, PA: John Benjamins.
There’s also some information in the highly entertaining book:
Burridge, K. (2005). Weeds in the Garden of Words: Further Observations on the Tangled History of English Language. Cambridge: Cambridge University Press.
 Strang, Barbara M.H. (1080). A History of English. London: Methuen.