The suffix -ly transforms words in two important ways.

First, it turns adjectives into adverbs, changing words that describe things into words that describe actions. When you add -ly to “quick,” you get “quickly,” which tells how something happens. A quick runner runs quickly. A careful student works carefully. A brave knight fights bravely. These -ly words answer questions like “How did it happen?” or “In what way?”

Second, -ly can turn nouns into adjectives that mean “like” or “characteristic of” that thing. A friendly person acts like a friend. A cowardly retreat is the kind of retreat a coward would make. Something kingly resembles or befits a king.

You'll find -ly everywhere in English because it's one of our most productive suffixes: it keeps creating new words as we need them. If someone invents a word like “zesty,” we can instantly understand “zestily.” Understanding this little suffix helps you decode thousands of words and create new ones when you need to describe exactly how something happened or what something resembles.