in prose the one must never be substituted for the other.
“Agreeably to my promise, I now write,” not “Agreeable to my promise.”
“An awful solemn funeral,” should be “An awfully solemn funeral.”
“He acts bolder than was expected,” should be “He acts more boldly.”
“Helen has been awful sick, but she is now considerable better.” “Helen has been very ill, but she is now considerably better.”
Do not use coarser for more coarsely, finer for more finely, harsher for more harshly, conformable for conformably, decided for decidedly, distinct for distinctly, fearful for fearfully, fluent for fluently.
Do not say “This melon is uncommon good,” but “This melon is uncommonly good.”
The word ill is both an adjective and an adverb. Do not say “He can illy afford to live in such a house,” but “He can ill afford.”
“That was a dreadful solemn sermon.” To say “That was a dreadfully solemn sermon” would more grammatically express what the speaker intended, but very or exceedingly would better express the meaning.
Such, So