which is correct, or both?

[tdye2]

I feel bad about that. or I feel badly about that.

[Coffa]

I feel bad about that. or I feel badly about that.

The correct sentence is "I feel bad about that". We are using the verb 'to feel' as a linking verb, and so must use an adjective. 'Badly' is an adverb, and so the only time when it is correct to use 'feel badly' is when we are modifying the verb 'to feel' - that is, our ability to feel is 'bad'.

To illustrate this more clearly, consider the example "I feel miserable" - we do not say "I feel miserably" because obviously the adverb 'miserably' would be a modifier for 'feel'. It is less clear with 'bad', and you will find a great many native English speakers make this error.

[matilda]

the first one is completely correct. but the second sentence sounds odd

[RonBee]

the first one is completely correct. but the second sentence sounds odd

It sounds odd because it is wrong. So don't feel bad about that.

[mykwyner]

The confusion comes from the fact that feel can be either a linking verb I feel sick or an action verb I feel a pebble in my shoe.

There are several English verbs like this, most of them have to do with the senses.

If your dog smells bad, he needs a bath.
If your dog smells badly, there is something wrong with his nose.

Look, sound, appear, and others all fit this category. I have an interesting exercise I do with my students where we make nearly identical sentences with dual-purpose verbs.

I guess this is my idea of fun.

Mike

[Humble]

Hello, Mykwyner
Would it be possible to see and copy this exercise of yours? Because it really is a vy common mistake – to use adverbs instead of adjectives.
Tnx