You’ve probably seen the phrase written both ways. This article discusses more important/ly as an introductory phrase. After reading it, you should have a good grasp of why more important is the best choice.
A copyeditor I spent good money on changed an introductory more important to more importantly in my manuscript. Later, I took a copyediting certification class, during which I learned that in such a case, “more important” should be used because the phrase is an adjective being used as a subject complement. (CMS uses the term subjective complement in 5.101).
Read the above graphic, then consider the following sentence:
“It’s nice to be important. More important, you should be nice.” (Not the most beautifully presented sentences, but I’m making a point.)
In the second sentence above, it’s understood that than that is follows more important.
“It’s nice to be important. More important [than that is] you should be nice. This helps put the expression into context, which in turn helps explain why it makes more sense to introduce the independent clause with more important than more importantly. Take a look at another example.
For safety reasons, a six-foot fence should be built around a pool. More important, the gate should be locked when unsupervised children are nearby.
Here, “more important” really means “more important for safety is [the gate should be locked]” Is, a linking verb, makes the subject and the subject complement (the comparative adjective more important) equal. In fact, the sentence could be reversed: The gate being locked is more important than the six-foot fence.
As a comparative adjective (CMS 5.84 explains that term) and subjective complement, more important ties one sentence to the other by making a comparison regarding the quality of safety.
Look up importantly in a dictionary, and you’ll find the word is an adverb that emphasizes a point or draws importance to an action.
“Chin up, he stepped onto the stage importantly, capturing the attention of his audience.” (How did he step on the stage? Importantly. It’s not very descriptive, but it is, nevertheless, an adverb.)
I could say, “He walked onto the stage more importantly than the other guy, who slouched and looked down.” However, getting back to the sentence about the fence and gate, I wouldn’t say, “More importantly, the gate should be locked when unsupervised children are nearby.” Technically, I could if I wanted to emphasize how the gate should be locked. However, that has nothing to do with my point, that one thing is more important than the other when it comes to safety.
If you use more importantly to introduce a sentence, consider how the expression relates to your previous statement. You want your sentences to flow in such a way that they form a cohesive, coherent thoughts and paragraphs.
Amy Einsohn and Marilyn Schwartz remind us in The Copyeditor’s Handbook that in The Elements of Style, Strunk and White prefer the adjective more important over the adverb more importantly as a “sentence modifier,” while Garner’s [Modern English Usage] thinks the preference is picayune (389-90). Sounds like Garner doesn’t think it matters which a person uses.
I believe Garner would agree with Strunk and White if he looked at this introductory expression on a micro level. If he did, he’d see the comparative adjective form, most important, should be used, as it’s working as a subject complement. “Sentence modifier,” as Einsohn and Schwartz describe more important/ly, is too general of a description to make the appropriate call on which version to use. Consider the following.
More importantly in the above statement suggests I should dare to act in a more important way than I dare to dream. How does one dare more importantly? Using more important instead of more importantly here would make more sense. Doing so would carry the message that putting action behind your dreams is more important than merely daring to dream. More important would connect the first clause to the second by working as a comparative adjective.
In Conclusion . . .
As a writer and editor, I prefer to know exactly why I’m using a particular form of a word or phrase. Therefore, I side with Strunk and White in the more important–more importantly debate. When used as a subject complement, or an assumed subject complement, the comparative adjective more important works best when introducing a sentence that relates to its predecessor by comparison:
“I had a great time on my vacation. More important [than that is], I caught up on sleep, which my body desperately needed.” Because the understood words are missing, a comma follows more important. If I had included that bracketed content, the comma wouldn’t be necessary.
We should have a good reason for our word choices. Use adverbs, such as importantly, where appropriate. However, when it comes to choosing between more important and more importantly as a “sentence modifier,” or to introduce a clause, your decision shouldn’t be based on whether an editor think it’s “picayune” to prefer one over the other. Approach the dilemma grammatically. Figure out the purpose of that modifier. Doing so will help you make a proper decision about which form to use. You don’t have to be an editor to figure it out, just a diligent wordsmith.