English fascinates me. Here is what I found on the topic...from grammarbook(dot)com:
<h1>Adjectives and Adverbs: When to use -ly</h1>Do you wonder when to add -ly to a word? For example, should you say, “He speaks slow” or “He speaks slowly.” What about, “He speaks slower than his brother.” Is this correct? Let’s find out.
Adjectives describe nouns and pronouns. They may come before the word they describe: “That is a cute puppy.” Adjectives may also follow the word they describe: “That puppy is cute.”
Adverbs modify adjectives, verbs, and other adverbs. If an adverb answers how and can have an -ly attached to it, place it there.
Example: She thinks slow/slowly. <em>Slowly</em> answers how she thinks.<br /> <br /> Example: We performed bad/badly. <em>Badly</em> answers how we performed.
Example: She thinks fast/fastly. Even though <em>fast</em> answers how she thinks, there is no such word as <em>fastly</em>.
Rule: When comparing, don’t drop the —ly. Simply add <em>more</em> or <em>less</em>.
Example: Answer the questions more quickly/quicker to win the prize.
Rule: English grammar has one tricky caveat that seems like an exception to these easy rules: If the verb is one of these four senses–taste, smell, look, feel–don’t ask how. Instead, ask if the sense verb is used actively. If so, attach the -ly. If the sense verb is not used actively, which is more common, don’t attach -ly.
Example: Roses smell sweet/sweetly. Do roses actively smell with noses? No, so no -ly.<br /> <br /> Example: The woman looked angry/angrily. Is the woman actively looking with eyes? No, only her appearance is being described.
Example: She feels bad/badly about the news. She is not feeling with fingers so no -ly.<br /> <br /> Example: She feels bad/badly since burning her fingers. She feels with her fingers here so the adverb (-ly form) is used.<br /> <br /> Pop Quiz
1. I feel bad/badly about telling that secret.<br /> 2. Walk slower/more slowly, please.<br /> 3. You look sad/sadly about the news.
Answers
1. bad<br /> 2. more slowly<br /> 3. sad
Woooooo, I wish I knew all that stuff. And I wish I could remember all that stuff.
Thanks
As a child, I used to soak those rules up like crazy...but you begin to question yourself as an adult. A review is always useful and fun.
Now, the next thing we need to do is read some Chaucer! Nothing like a bit of Middle English! (It's incredible to realize how much of our language originates from the German and Dutch languages.)
Why? What did I do to you that you would punish me like that? 
