Here's why:
* Adjectives describe nouns or pronouns (people, places, things, ideas).
* Adverbs describe verbs, adjectives, or other adverbs. They answer questions like "how," "when," "where," or "to what extent."
To use "friendly" as an adverb, you need to use a related word:
* Friendly (adjective): "The dog was friendly."
* Friendlily (adverb): "The dog greeted us friendlily."
Examples:
* Adjective: "She gave me a friendly smile."
* Adverb: "She greeted me friendlily."