Should I use "important" or "importantly" in the introductory phase of a sentence?

Question

Should I use important or importantly in the introductory clause of the second sentence:

Appellant's argument is premature. More important, it lacks merit.

Google NGram shows a significant preference for "more important." Recently, however, it appears the democracy of usage has swung slightly in favor of "more importantly."

I prefer the former, but I am more concerned about learning which one is grammatically proper, and the particular rules that control the analysis. Can someone walk me through it?

Answer 1

Appellant's argument is premature. More importantly, it lacks merit.

"Important" is an adjective, used to qualify a noun.
"Importantly" is an adverb, typically used to qualify a verb or an adjective.

Your context requires an adverb, not an adjective.

Additionally, I would use "it" rather than repeating "the argument".

Answer 2

This is one of those cases where ngrams don't tell the whole story. Words and phrases can be used in multiple ways, so just seeing how often they appear in ngrams doesn't help. The correct form in your example is more importantly, but the ngram will register hits for "more important" in sentences like this:

I considered reading a book after lunch, but I decided running some errands was more important.

Just because more important is correct somewhere else and registers ngram hits doesn't mean it's correct in the way you want to use it.

So, yes, more importantly is what you want to use in your example sentence:

More importantly, it lacks merit.

I'd also take note of your first sentence; this sounds a bit like legal jargon, and so in that case maybe it is appropriate to refer to the appellant as a proper noun (without article). But if this isn't acceptable legalese, you're missing a the at the beginning of the sentence:

The appellant's argument is premature. More importantly, it lacks merit.

Answer 3

NGrams does not show a significant preference for "More important," if we search for a text string more specific to OP's context...

Having said that, what the NGram does suggest is that the "ungrammatical" form has been rising in popularity over recent decades. But if OP is concerned to learn and use "strictly valid" grammar, he should use the adverbial form more importantly.

That's because the presence or absence of the word more makes no difference to the formal grammar. You do something (lack merit, in this case) adverbially. Nothing changes if you do it more adverbially.

If you consider the situation without the word more...

Appellant's argument is premature.
**Important, it lacks merit.*

...all native speakers will find this unacceptable (as indicated by the asterisk). But even though in principle the word more makes no difference to the grammaticality, informal (and/or uneducated) speakers/writers do in fact often use OP's version.

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One could say there's an implied/elided "What is more" before important (in practice people often would include either that or an equivalent construction). But OP specifically asks what form he should use; it's not strictly necessary to explain why even native speakers often disregard logic/grammar in this context.

Answer 4

Although it may be ungrammatical, this use of "More important" seems to be quite common (the cited authors reside in the US):

"More important, it appears that theory-based approaches that are tailored to specific populations and behaviours can be effective in different cultures and communities." (source)

"More important, it provides information necessary to explore the relationship of experiencing violence to health status and to the use of health services..." (source)

"Most important, it turns out that how schools hire and how they use their resources can make a major difference." (source)

"Perhaps more important, it may take the form of information useful in trading in the company's shares..." (source)

Answer 5

The adverbial phrase "more importantly" is correct because the lacking of merit is being modified, not the pronoun it (or its antecedent, the appellant's argument).