Optional flat adverbs

In case of the following adverbs, there is often not much agreement among native speakers whether the flat versions may be used at all, or in what context or "level of style" - even if they are real flat adverbs and their usage is grammatically correct. For the non-native speaker, often the use of the regular form will be the safe choice. However, there are cases of fixed phrases in which the flat version is mandatory, or very short sentences, in which the regular form will seem bulky and out-of-place.

bad, bright, cheap, clean, deep, different, healthy, kind, loud, quick, safe, slow, sure, tight

bad [optional] - Though "bad" is originally not a flat adverb and should never be used as one in British English or written language, it is quite frequently used as a flat adverb in spoken language by Northern Americans. This holds true especially for certain phrasings that occur more often - e.g. "to do bad", "to hurt bad", "to want sth. bad", "to treat sb. bad", or "to talk bad about sb."
Additionally, there are phrasings that are used to express "become bad", e.g. "go bad" or "turn bad", which means that in these cases what may look like an adverb is in fact an adjective.

Examples I came across:
1) "North American informal": "He wants kids really bad." -- "My stomach started hurting really bad." -- "They should make sure people don't treat you bad." -- "My TAs are complaining how bad we did." -- "She gets on my nerves so bad."
2) In fact an adjective: "The food will go bad if we don't eat it today." -- "If you make a mistake, things go bad." -- "Don't turn this good heart bad" by the American band "Biters" -- "Fairytale gone bad" by the Finnish band "Sunrise Avenue"

bright [optional] - "bright" and "brightly" mean the same, are both in use and seem both widely accepted. I found less emotional discussion about the usage of "bright" as a flat adverb than in the case of other optional flat adverbs. One could argue though that this acceptance is due to the fact that one might say that a star that shines bright actually is bright, so that some people feel that it might in fact be an adjective.
mentioned in "Flat adverbs are flat-out useful" and "When adverbs fall flat"

cheap [optional] - "cheap" is possible as a flat adverb in phrases such as "They were selling these items off cheap" or "I only bought these saucepans because they were going cheap" (#54). In the latter case, one could argue that "cheap" serves in fact as an adjective. Usually, the use of "cheaply" will be the safe choice. (E.g.: The product seems to be cheaply made - they'll have to sell it cheaply.)
mentioned in "When adverbs fall flat"

clean [optional] - "clean" is a difficult case - native speakers do not always agree on whether the use of the flat or the regular form sounds "more natural". Additionally, there are the usual differences between British and American English and different levels of style.
In the idiomatic expression "to come clean" the word has got a somewhat different meaning than e.g. in the literal phrase "Come cleanly shaved!" There is also the composition "clean-cut", meaning "straight and simple" (e.g. architecture) or "neat and attractive" (e.g. appearance, style, or clothing).
taken from "Flat adverbs are flat-out useful"

More examples:
1) Cambridge Dictionary
The election campaign was not conducted very cleanly.
The plate broke cleanly in half.
Opinions were split cleanly between men and women.
2) Google
"he vaulted cleanly through the open window"
"the engine burns very cleanly"
An American native speaker commented that he'd prefer "the engine burns very clean" and "I broke the plate clean in half" (#59).

deep [optional] - Both versions are possible ("breathe deep" and "breathe deeply" are identical in meaning), but there is also a distinct idiomatic usage that requires the flat form: "to go deep".
In most longer sentences and in written language, the regular form seems the safer choice.
As a submodifier, the regular form has to be used: deeply impressed, deeply worried...
On LEO, "breathe deep" and "go deep" are accepted by an American, but rejected by a British native speaker (#55+56). The British band "Paradise Lost" uses the phrasing "wounds cut deep" in one of their songs.
taken from "Flat adverbs are flat-out useful"

different [optional/ doom] - In spite of the Apple campaign "Think different!", the regular form "differently" seems to be more widely accepted. Thus, as a rule of thumb, "differently" clearly seems to be the safer choice. However, the choice between "different" or "differently" may not only depend on the style but possibly also on the context, as explained by an American native speaker in the following two examples on the LEO forum:

1) "He thinks different than I do (on that issue.)" (We disagree on that issue.) In contrast, "He thinks differently than I do" might mean, for example, that he reads everything he can about the subject and then prays about it, etc., before making a decision -- whereas I simply ask the neighbor boys what they know about the subject and then do whatever they recommend.
2) In my day-to-day (informal) communications, I'd usually say "He acts different than me"-- if I mean that his behavior is different from mine. Of course, I'd say "He acts differently than I do"-- if I mean that we are both actors and we have different acting methods or styles.
mentioned in "Flat adverbs are flat-out useful"

healthy [optional] - "We all know we should eat healthy!" -- "Children should be encouraged to eat healthily." Both versions are valid, and they mean the same. Some people consider "healthily" the better or even the only acceptable choice, e.g. a British native speaker on the LEO forum. There is in fact a lot of discussion going on on the internet, as can easily be seen when searching the web for "healthily". There is, e.g., a short article by the Czech jack-of-all-trades Jakub Marian who furiously rejects the use of "healthy" as an adverb, and a much more detailed article by the American blog Grammarphobia stating that, in the end, resistance will be futile and "healthy" will be accepted as an adverb as it is already widely in use. Until then, for the non-native speaker who doesn't want to stir up a hornets' nest, "healthily" might be the better choice.

kind [optional] While "kindly" is mostly seen as the only correct version, certain phrasings like "treat me kind" are widely used, especially in a less formal context and in many song texts. The British animal charity "Treat me kind" shows that the phrasing is not limited to American English.
"Kind" and "kindly" can have slightly different roles: "Treat her kind" (i.e. "friendly"), but "Think kindly of her." (i.e. well-disposed). "kindly" itself can also be an adjective (#54).

More examples and discussions:
Users on UsingEnglish and LEO insist that "kindly" is the grammatically correct version and "kind" as an adverb only works in an informal context.
"Treat me kind" is the title of songs by the US-American country singer Goldie Hill, the Greek band "The frantic five" and the Canadian band "The Harpoonist & the Axe Murderer". It is also a line in Mariah Carey's song "Vision of Love". Goldie Hill also offers the line "You don't mean to treat me unkind" (World I can't live in), and Whitney Houston sings "I hope life treats you kind" (I will always love you).

loud [optional] - Both versions are possible, and it seems that the usage depends not only on style but also on the metrics of the sentence. A short sentence as in "Can you hear me?" - "Loud and clear!" will not be burdened with the regular form.
However, I feel that in some of those cases not only the metrics may play a role but also - as so often - the question if one could actually interpret the assumed adverb as an adjective: In the case of "knock loudly", it is clearly the knocking that is loud, so the adverb makes sense. In case of "I can hear you loud and clear", though, one could maybe argue that "clear" indeed describes the hearing capacity, but this most obviously does not work for "loud". In fact, it seems more likely that both ostensible adverbs are in fact adjectives describing the quality of the perceived sound.
Apart from those difficult cases, there are certain fixed phrasings such as "to call out loud, to think out loud, and to laugh out loud" - at least in American English.
In many other cases, however, the regular form is used: "The four men laughed, joked and talked loudly." -- "He loudly blows his nose." -- "Please knock loudly!"
An interesting case is what a monster in an American online game once said to me: "Now, let us see how loud you scream!" An American friend commented that "though 'loudly' is correct, nobody would ever use it in that particular sentence". So in that case, the "flow"/ the "metrics" of the sentence seem to be the decisive factor.
mentioned in "When adverbs fall flat", some examples taken from Cambridge Dictionary

quick [optional] - This flat adverb is interchangeable with its normal equivalent: "Come quick" and "Come quickly" mean the same thing. It also works for the comparative degree: "more quickly" or "quicker" are both fine. (Here, an American native speaker suggests the flat form in an unrelated context.)
mentioned in "When adverbs fall flat" and "Flat adverbs are flat-out useful"

safe [optional] - There was (or maybe still is) the street sign asking car drivers to "Drive safe!", which caused discussion about the usage of "safe" as a flat adverb, showing that though grammatically correct, it is not accepted by everyone. But there are fixed expression such as "to play it safe" (also seen on a street sign on the Furka pass in Switzerland in 2018) which would sound odd if the regular adverb was being used. In case of "to keep something safe", however, one can argue though that "safe" is in fact an adverb describing how the item in question be will be: safe.
In most cases when there is clearly a need for an adverb, "safely" will be the the better choice, as "safe" as a flat adverb is rejected/ dubbed "informal" by some native speakers.
mentioned in Merriam Webster and "When adverbs fall flat"

slow [optional] - Slow and slowly are interchangeable: "Drive slow" and "Drive slowly" mean the same thing. There are fixed expressions such as "let's take it slow" in which the regular form would sound odd. However, as so often, for most other cases, the flat form is considered "informal" or "mainly American" by many native speakers, and the regular form has thus to be recommended as the "safe choice" for the non-native speaker (see also: #55-56).
taken from "Flat adverbs are flat-out useful"; also mentioned in "When adverbs fall flat"

sure [optional] - Both "sure" and "surely" can be used as an adverb roughly meaning "certainly". "Sure" as an adverb is considerd "North American informal" by Oxford dictionaries: "Texas sure was a great place to grow up" - "Are you serious?" "Sure."
mentioned in "When adverbs fall flat"

tight [optional] - "tight" as a flat adverb generally can only follow its verbs and cannot modify anything other than a verb. It has been fossilized in idioms, such as "sit tight" and "sleep tight".
The British pop band East-17 had a song called "Hold my body tight."
Regular form: Her eyes were tightly closed. He held on tightly to her arm. Tightly knit.
taken from "When adverbs fall flat"; some examples from Oxford Dictionaries

Innocent adventurer being threatened by a flat adverb.