True flat adverbs

If the word of your choice appears in this list, you are lucky - you don't even have to know if it is used as an adjective, adverb or submodifier in your case, it will just always look the same.
As the regular form of these adverbs simply doesn't exist, there won't even be any discussion about the style or context.

daily, early, far, fast, freezing, ill, long, low, soon, straight, tough


freezing: "freezing" is indeed a true flat adverb - there is no such thing as "freezingly". However, I have come across only one way of using it: As a submodifier in "freezing cold".

ill: "ill" as an adverb exists mostly in compositions such as ill-advised, ill-assorted, ill-bred, ill-conceived, ill-defined, ill-disposed, ill-equipped, ill-mannered, ill-minded, ill-fated, ill-fitting... ill-informed... or ill-treat. These compositions are usually listed as independent words in the dictionaries.

A few other possibilities are listed in the Cambridge Dictionary:
"ill" can mean "in an unkind way" in the phrases "to speak ill of so." (formal/ old-fashioned) and "to treat somebody ill" (marked as literary). "to bode/ augur ill" means to be a sign of bad things in the future.
"ill" can also mean "hardly" in the phrasing "so. can ill afford sth."
On LEO, the archaic use "he dealt ill" as in "he behaved badly" is mentioned.

low: According to the native speakers on the LEO forum, "low" is a true flat adverb and "lowly" an independent adjective. In Oxford Learner's dictionary and in the Cambridge dictionary, "lowly" indeed only exists as an adjective. The entry in Oxford Dictionaries, however, also has lowly as an adverb and was rejected by all native speakers who commented on it (#29-35).

Examples of "low" as an adverb:
"You didnít have to stoop so low" (fixed phrase). "Creep low, fly high" (children's book about insects), "Fly high, fly low" (children's book about a pidgeon). Bow low. A worker on low pay, a low paid/ low-paid worker (#29). To a low degree; in a low manner.
Instead of "said lowly", the phrasings "whispered softly" or "said in a low voice" were recommended by a Briton and supported by a New Zealand, an American and another British native speaker (#34+35, #40+41).

tough: "tough" is apparently not used much as an adverb. The only usage I came across is "Hang tough" (seen e.g. in an American online game; marked "US informal" by Cambridge Dictionary). It was supported by an American, but rejected by a British native speaker (#55+56).