“Most important” and “most importantly” are both correct in modern US English, but they do not always perform the same grammatical job.
The easiest way to choose is to look at what the phrase does in the sentence. Most important is based on the adjective important. Most importantly uses the adverb importantly.
That difference matters most when one form comes directly before a noun. It also explains why both forms can sometimes introduce a sentence.
Quick Answer
Use most important when it describes a noun or acts as an adjective complement:
The most important factor is timing.
Use most importantly when it introduces or emphasizes a statement:
Most importantly, check the information before submitting it.
However, Most important, check the information before submitting it is also acceptable. It is an introductory, shortened construction rather than an ordinary adjective directly modifying a noun.
So the answer is not “always use one.” Both forms are correct, but their grammatical structures differ.
Why People Confuse Them
The confusion comes from the fact that the two expressions have almost the same meaning.
Both can signal that something has the highest importance. The visible difference is only the ending -ly, but that ending changes the grammatical role.
Compare:
The most important point is accuracy.
and:
Most importantly, accuracy matters.
In the first sentence, important describes point. In the second, importantly introduces the statement that follows.
The confusion becomes greater because most important can also appear at the beginning of a sentence:
Most important, stay calm.
That does not make the two forms identical. It means English allows more than one grammatical construction for expressing the same broad idea.
Key Differences At A Glance
| Context | Best Choice | Why |
|---|---|---|
| Before a noun | most important | It describes the noun |
| After a linking verb | most important | It functions as an adjective |
| Introducing a key statement | most importantly | It functions adverbially |
| Introducing a key statement in a concise style | most important | This introductory construction is also acceptable |
| Before a noun as an adjective | most important | “Importantly” cannot directly modify the noun |
Compact comparison
- Most important → adjective phrase; commonly describes a person, thing, factor, reason, point, or issue.
- Most importantly → adverbial expression; commonly emphasizes the statement or point that follows.
Meaning and Usage Difference
The basic meaning of both expressions relates to the highest level of importance.
The main difference is grammatical.
Most important
Most important commonly modifies a noun:
The most important decision is still ahead.
It can also follow a linking verb:
Accuracy is most important here.
In these examples, important has an adjective function.
Most importantly
Most importantly commonly emphasizes an entire clause or statement:
Most importantly, the customer needs a clear answer.
It can also appear inside a sentence:
The plan is affordable and, most importantly, practical.
Here, importantly is functioning adverbially.
A useful structural test
Ask what comes immediately after the phrase.
If you have:
the most important reason
choose most important.
If you have:
Most importantly, we need to act
the adverbial form fits naturally.
The sentence-opening form Most important, we need to act is also possible, so the choice there is not simply a matter of “right versus wrong.”
Tone, Context, and Formality
Neither form is automatically more formal.
Most important can sound concise and direct:
Most important, make sure the address is correct.
Most importantly can sound slightly more explicitly connected to the statement that follows:
Most importantly, make sure the address is correct.
Both are suitable for professional, academic, and everyday writing when used appropriately.
Some traditional writing advice has criticized sentence-level uses of most importantly. Modern usage guidance does not support treating that objection as a universal grammar rule.
For US readers, the practical point is simple: do not reject most importantly merely because it ends in -ly.
Which One Should You Use?
Use most important when you are clearly describing a noun:
This is the most important step.
Trust is the most important quality in a manager.
Use most importantly when you want to emphasize a complete statement:
Most importantly, save a copy of the document.
Most importantly, the customer deserves an honest answer.
You can also use most important as an introductory expression:
Most important, keep the original receipt.
That construction is correct, although some writers may prefer the more explicitly adverbial most importantly.
A reliable editing habit is to identify the grammatical job rather than automatically adding or removing -ly.
When One Choice Sounds Wrong
Most importantly sounds wrong when you use it as though it were an adjective before a noun.
Incorrect:
The most importantly reason is cost.
Correct:
The most important reason is cost.
Incorrect:
Safety is the most importantly concern.
Correct:
Safety is the most important concern.
The problem is not the idea of emphasizing importance. The problem is grammatical function.
On the other hand, most important is not automatically wrong at the beginning of a sentence:
Most important, review the contract carefully.
Calling that construction incorrect simply because most importantly is also possible oversimplifies the issue.
Common Mistakes (and Quick Fixes)
Mistake 1: Putting “most importantly” before a noun
The most importantly factor is price.
Fix:
The most important factor is price.
Mistake 2: Assuming every sentence opening requires “-ly”
Most importantly, keep your phone charged.
This is correct, but so is:
Most important, keep your phone charged.
The two constructions differ in grammatical form, not in basic importance.
Mistake 3: Treating style preference as a grammar rule
Some writers prefer most important because it is shorter. That preference does not make most importantly universally incorrect.
Mistake 4: Adding “the” before the adverb
Do not write:
The most importantly, check the figures.
Instead:
Most importantly, check the figures.
The article the belongs to noun phrases such as:
the most important figure
Everyday Examples
Here are practical examples showing how the forms work in ordinary US English.
Before a noun:
The most important part of the interview is preparation.
After a linking verb:
Being honest is most important when dealing with customers.
At the beginning of a sentence:
Most importantly, confirm the appointment before you leave.
An alternative introductory construction:
Most important, confirm the appointment before you leave.
Inside a sentence:
The new plan is cheaper and, most importantly, easier to manage.
With a noun after the adjective:
Her most important goal is to finish the project on time.
With an emphasized statement:
Most importantly, everyone needs to understand the safety instructions.
These examples show why the choice should depend on sentence structure rather than a blanket rule.
Dictionary-Style Word Details
Verb
Most important: Not commonly used as a verb. It is an adjective phrase based on important.
Most importantly: Not used as a verb. It is an adverbial expression based on importantly.
Noun
Neither expression is itself a noun.
Most important can be part of a noun phrase:
the most important decision
Most importantly does not directly modify a noun:
Most importantly, the decision was fair.
Synonyms
Most important: Closest plain alternatives include most significant, most essential, and highest-priority, depending on context.
Most importantly: Closest plain alternatives include above all, above all else, and most significantly, depending on the intended meaning.
These alternatives are not interchangeable in every sentence, so the surrounding context still matters.
Example Sentences
Most important:
The most important issue is customer safety.
Accuracy is most important in this report.
Most importantly:
Most importantly, keep the original records.
The service is fast and, most importantly, reliable.
Word History
The two expressions come from the adjective important and its related adverb importantly. The modern usage issue is mainly grammatical and stylistic rather than a difference in basic meaning.
Traditional style advice has objected to some sentence-level uses of importantly, while modern references recognize such uses. There is no need to invent a sharp historical dividing line between the two forms.
Phrases Containing
Common constructions include:
- the most important thing
- the most important reason
- the most important factor
- the most important part
- most importantly, …
- and most importantly, …
- but most importantly, …
- most important, …
The first group normally uses most important as part of an adjective phrase. The second group uses most importantly as an introductory or parenthetical expression.
FAQs
Is “most important” correct in standard US English?
Yes. “Most important” is standard when important functions as an adjective, especially before a noun or after a linking verb. It can also appear as an introductory expression: “Most important, review the final numbers.” It is not an outdated form.
Is “most importantly” also correct?
Yes. “Most importantly” is a standard adverbial expression. It commonly introduces or highlights the point the writer considers most significant: “Most importantly, the patient needs rest.” You do not need to avoid it simply because some older usage advice criticized the -ly form.
Can “most important” begin a sentence?
Yes. “Most important, we need to protect the customer’s information” is grammatical. Here, the phrase works as a supplementary expression rather than directly describing a noun. It is different in structure from “Most importantly, we need to protect the customer’s information.”
Can “most importantly” describe a noun?
No. Use “most important” before a noun: “the most important reason,” not “the most importantly reason.” “Importantly” is an adverb, so it does not directly modify a noun in that construction.
Are “most important” and “most importantly” interchangeable?
Not everywhere. They can express a similar idea when used to introduce a key point, but their grammatical roles differ. “The most important factor is cost” describes a noun, while “Most importantly, consider the cost” emphasizes the statement that follows.
Should I avoid “most importantly” in formal writing?
No. “Most importantly” is acceptable in formal US English. Some traditional style advice prefers “most important” in introductory uses, but that is a style preference rather than a universal grammar rule. Choose the form that matches your sentence structure and intended emphasis.
Conclusion
Most important and most importantly are both correct in US English, but they have different grammatical roles.
Choose most important when it functions as an adjective, especially before a noun:
the most important reason
Choose most importantly when it functions as an adverbial expression that emphasizes a statement:
Most importantly, check the facts.
At the beginning of a sentence, most important can also be correct:
Most important, check the facts.
The best choice depends on the structure of the sentence, not on a simple rule that one form is always right and the other is always wrong.
