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.”