It's pretentious (and incorrect) to say, "I felt badly."

 Hi! I know I'm preaching to the choir, but I think well-meaning students (and many well-meaning adults) say, "I felt badly" instead of, "I felt bad." This is how I might try to help. I’d tell them that it's just like, "I feel full," or, "I feel pretty." You don't feel "fully" when you've eaten too much. When people say, "I feel badly," they are trying to make "bad" an adverb, which is certainly understandable. But they just don't have to; actually, they shouldn't. 

I read recently that Donald Trump said he "felt badly" for the Cornell Seven. It's a grammar error and not his first. Who cares about the grammar error? It's what he "felt badly" about that is egregious.

I actually don't give a shit if people say, "bad" or "badly" with "I feel." It's truly more important what they feel bad about and why. 

So next time, let people off the hook and tell them that it's ok to feel sad (instead of sadly).

Again, I would never call someone on a grammar error in conversation--I'm too distracted by what they're trying to share! And it's mean and pretentious.