Examples
Correct usage of
more importantly: "The duchess is carrying herself more importantly than the other attendees at this party. It seems that she's determined to display a deportment befitting her role as the dignified host of this gathering."
Note: In this sentence, the adverb part, "more importantly" modifies the verb "
carrying herself."
Correct usage of
more important:
"Enough of that line of thinking! (A) more important (matter) is that we must make certain that all of the food is presented to the duchess's guests in a timely manner." I've put parentheses around the words that can be omitted from that sentence, so "A more important matter" becomes "More important," and then add a comma, which is the correct way to use that
comparative transition.
A notable exception is if you remove the word
more. In that case, the initial word "importanly" modifies the entire rest of the sentence or phrase that follows.
For example, you can say, "Importantly, all of the glasses, plates, and silverware are sparkling," but
not "Important, all of the glasses, plates, and silverware are sparkling."
In the second instance, it can be grammatically correct if written as a notification or pronouncement, using a colon. "Important: All of the glasses, plates, and silverware are sparkling." Although that changes the meaning and intent.
The main point is that "more important" should be used for comparative purposes. It is used when something is previously mentioned and the speaker wants to then point out another matter which is of greater importance than the previously mentioned matter(s). When a sentence is started with only "importantly" there is no implicit comparison with a previously mentioned matter, thought, or idea. Using "importantly" without "more" to start a sentence just denotes that the words following "importantly" have been deemed important, but not necessarily
more or less important than other matters. Again, when no comparison is being made, using
importantly, by itself, is correct.
This probably helps to explain how "more importantly" came into widespread (albeit, incorrect) use.
I can only hope that this impromptu lesson in English helps someone in the future.