as a noun, 'bad' refers to something that is undesirable or of poor quality. It's less common than the adjective or adverbial usage.
There's good and bad in every situation.
We need to eliminate the bads and focus on the positives.
The city has its share of bads, but there's a lot to love about it too.
the noun form of 'bad' is less common and might sound informal or poetic in certain contexts.
as an adjective, 'bad' describes a noun, indicating that something is not satisfactory, not of good quality, or undesirable.
She had a bad day at work.
The milk smells bad; I think it's spoiled.
He made a bad decision that affected everyone.
as an adverb, 'bad' modifies verbs, often indicating that something is done poorly or ineffectively. However, it's worth noting that this usage is colloquial and considered non-standard. The standard adverbial form is 'badly.'
He played bad in yesterday's game. (Colloquial; standard form would be 'He played badly...')
I feel bad about forgetting her birthday. (Note
In this context, 'feel bad' is standard and correct, as 'feel' is a linking verb here, and 'bad' is an adjective describing the subject's state.)
Remember that using 'bad' as an adverb is colloquial and might not be suitable for formal contexts. In standard English, 'badly' is the preferred adverbial form.