What’s the visual difference between an adjective and an adverb? Well, all adverbs end in -ly, and no adjectives do, right? Wrong on both counts.
Some adverbs, called flat adverbs, lack the -ly suffix. The words in this category, like straight, can be used both as adverbs (“Drive straight through the intersection”) and adjectives (“He drew a straight line”).
Some adverbs, like slow, are interchangeable with their -ly forms (see this post). Others, like hard, are distinct in usage from the adverbial -ly form (“I worked hard” is opposite in meaning from “I hardly worked”) and have no adjectival -ly form.
At the same time, dozens of adjectives end in -ly, and many have no adverbial equivalent. (Some adjectives that pertain to periods of time, such as daily, weekly, and monthly, can be both adjectives and adverbs: “They followed a daily regimen”; “They followed the regimen daily.”) For example, friendly can modify a noun (“She wore a friendly smile”), but although one can write, “She looked friendly,” it means she had a friendly look, not that she looked at someone in a friendly manner. (Friendly is not an adverb; friendlily is, though I’ve never seen it used, and one must otherwise use the adverbial phrase in the previous sentence, or a similar one, in place of friendly.)
Other adjectives ending in -ly that are not also adverbs include costly (“It was a costly event”), miserly (“That’s a miserly attitude”), and unruly (“He’s an unruly boy”). One cannot, for example, act costly, miserly, or unruly; one is said, for example, to spend in a costly manner, to behave in a miserly fashion, or to engage in unruly behavior.
Adjectives ending in -y can be converted to adverbs with the insertion of -il- before the final letter, as in frosty (“She gave him a frosty look”) becoming frostily (“She looked at him frostily”), but, friendlily notwithstanding, few adjectives can be converted to adverbs in this manner. For example, “She gave him a deadly look” is correct, but “She looked at him deadlily is not.”
An example I use is the phrase ‘almost unseemly’ which neatly contradicts the ‘rule’ in the first sentence.
From a linguistic standpoint, it would be wrong to assume that by merely looking at a word you can tell of it is an adjective or an adverb. The syntax alone would determine if the word is an adjective or an adverb. Since English butchers adverbs by allowing slow and slowly to be interchangeable and dare we forget the difference between good and well! 🙂
Morphology vs. Syntax is the better way to look at this question.