What is the correct usage? Apparently it is "I feel badly", but but wouldn't that mean you have an inadequate ability to feel?
Assuming you're talking about a situation where something bad has happened to your friend, and you're saying you feel unhappy on their behalf, then "I feel bad for you" is correct.
You are right that "I feel badly" would mean you are having difficulty in feeling at all - which would be a rather unusual thing to say :)
In general the verb "to feel" will take an adjective (happy, sad, good, bad, angry, relieved) after it to indicate the feeling, rather than an adverb.
Certain verbs like feel, smell, and taste take adjectives as complements, not adverbs. If you use an adverb with them, it changes the sense altogether, because it now modifies the verb rather than serving as a predicate complement describing the subject.
- Sour milk tastes bad. Honey tastes good.
- I feel bad that I didn’t go. I feel good about that.
- Those flowers smell good. That sewer smells bad.
- He tastes poorly because he’s burnt his tongue.
- My fingers feel badly when I have gloves on.
- A man with no nose smells poorly if at all.
So you can have a dog that smells bad but like all dogs, he nonetheless smells well.
Badly is an adverb and explains how something was done.
She was hurt badly.
Bad is an adjective and describes a noun.
She is a bad driver.
Since the word being modified is "feel" (a verb), the proper modifier is "badly" (an adverb).
This is essentially the same as the difference between using "good" and "well".
- I feel well.
- I am in good health.
Well modifies the verb feel while good modifies the noun health.