These words are often confused.
Bad is an adjective whereas badly is an adverb.
When someone feels ill, they feel ‘bad’, not badly. Likewise, when someone feels sorry about something, they feel bad, not badly.
- Don’t feel bad about breaking the glass.
- I feel bad. I think I should see a doctor.
The only meaning of badly is ‘improperly or in an unsatisfactory manner’.
- My children always behave badly when we have guests.
- You sing very badly.
‘Badly’ can also mean very much.
- She wants a new car badly.