An adverb is a word that modifies a verb or another adverb, or perhaps an adjective — or possibly even a clause or an entire sentence. How versatile! But there’s more to this part of speech: It can sometimes shed the nearly ubiquitous -ly ending and, though it subsequently appears to be an adjective, retains its adverbial function.
The most notorious instance of this transformation was the 1997 Apple Computer ad campaign that urged people to “Think different.” Oh, the uproar from uptight grammarians! (Followed by a quieter “Get over it” from — ahem — more open-minded observers.) Not only has different been attested in adverbial use for hundreds of years, but many other similar terms are part of the language (and they used to be even more common than they are now).
Some flat adverbs have no normal adverbial form (that is, one ending in -ly): Straight is one example. Others have a normal form, but the two forms have distinct meanings (“Jump high,” but “I think highly of her”). Still others are interchangeable. (“Hold on tight” and “Hold on tightly” mean the same thing.)
Here are some other flat adverbs; note how they’re most often suitable for brief imperative sentences (those in which the writer is issuing a direction or a command):
1. Bright: This word is interchangeable with brightly in sentences such as “The stars shine so bright on moonless nights.”
2. Clean: This usage is distinct from the -ly form: The idiomatic expression “Come clean” doesn’t have the same sense as the literal phrase “Come cleanly shaved.”
3. Close: The flat form and the normal form have related but different meanings: “Keep close,” but “Keep closely arrayed in formation.”
4. Deep: This term can be interchangeable with the -ly form (“Breathe deep” and “Breathe deeply” are identical in meaning), but it also has a distinct idiomatic usage: “Go deep.”
5. Far: This flat adverb has no -ly form: “You will go far in life.”
6. Fast: Fast is another flat adverb with no normal equivalent: “Run fast.”
7. Flat: The flat and normal senses of this term are similar but distinct: “I was turned down flat,” but “I was flatly refused.”
8. Hard: Hard and its -ly form are highly distinct in meaning: “I hit it hard” is almost the opposite of “I hardly hit it.”
9. Kind: Kind and kindly have slightly different roles: “Be kind,” but “Think kindly of her.”
10. Quick: This flat adverb is interchangeable with its normal equivalent: “Come quick” and “Come quickly” mean the same thing.
11. Right: Right and rightly have different senses: “Do right,” “Stay right there,” or “He aimed right for the target,” but “You are rightly upset.”
12. Sharp: Sharp and its normal form are interchangeable (“Dress sharp,” or “Dress sharply”), but there’s also a distinct flat-form meaning: “Show up at eight o’clock sharp.”
13. Slow: Slow and slowly are interchangeable: “Drive slow” and “Drive slowly” mean the same thing.
14. Soon: This flat adverb has no -ly equivalent: “Come again soon.”
15. Tough: This adverb is also without a normal version: “Hang tough.”