Do you know how to tell them from adjectives?
Some words that end with –ly aren't adverbs, of course: “family” and “elderly,” among others. “Family” is a noun that can be used as an adjective: a family outing. “Elderly” is most commonly an adjective--the elderly daredevil--but it can be used as a collective noun as in this phrase: caring for the elderly.
In the case of the word “bodily,” the –ly suffix turns a noun (“body”) into an adjective, “bodily,” as in “bodily functions”; but “bodily can also be used as an adverb, as in “Cindy removed Bruno bodily.”
On the other hand, if you tack an –ly onto the noun “ear,” you get “early,” which can be an adjective and adverb, but has nothing to do with hearing – unless your parole hearing is early, I suppose.
You can see from these variations and similarities how confusion can arise—easiLY.
Writers often clutter their text by tossing in a superfluous –ly, often because common parlance has superseded proper usage. That is, people’s speech—which by nature is more casual—takes hold in even the most formal writing.
Say you’re writing a letter, memo, or article with several elements, and you want to offer them in serial form. Begin the paragraphs (or sentences, for shorter elements) with “first,” “second,” and “last”—NOT “firstly,” “secondly,” and “lastly.”
Let’s let the word “first” set the standard for the rest; the usages are the same. First can be an adjective—the first man on the moon—or an adverb—phone first if you’re coming to visit.
You might write or say: “First, the goal of this project is to increase sales of our sardine cookies.” (And good luck with that.)
Here, “first” is basically shorthand for “the first point is.” What follows is a noun; in this case it’s a big noun, a nominative clause—the goal of the project—but it still behaves like a noun, so its modifier is an adjective: first.
One could argue that by “first” you mean “in the first place,” so it would be an adverb. Fair enough, but either way you don’t need “firstly”; “first” will be just fine.
Is “firstly” a word? Well, sure, it’s in the dictionary. But if “first” can be used as an adverb, why the heck would you need or want to slap an –ly suffix on it? Spare your fingers the extra keystrokes, your mouth the extra syllable, and your audience the clunkiness of “firstLY.
What about "most important"? Or should that be "most importantly"?