If you use improper grammar, do you write bad...or badly? Know the difference, and you can at least change the descriptor. Bad and -ly If you've heard the song, you know what the word means. Bad in slang can be used to describe any number of positive opinions and feelings. In more formal usage, bad usually denotes something that is undesirable. In grammar, it's a bomb waiting to explode ugliness all over your writing. The problem is, bad has a friend named -ly , and you never know when or where he's going to show up...or if he's supposed to be there. When -ly shows up to your word party, he changes everything. He attaches to bad to become badly , and that's a whole different part of speech altogether. Now, the word is an adverb, a word that's used only to modify another adverb, a verb or an adjective.What does that mean? It's simple: it means badly can only mingle with another word at the party. If I say I was badly and end the sen...