It certainly can be correct. As with all grammar it depends on how it is used
Yes, "she sings badly" is grammatically correct. The adverb "badly" is describing the verb "sings" in this sentence, indicating the quality of her singing.
hrtytrytryr
yes it is a real word. It is also correct grammatically and more stupid is not correct.
No, "really badly" is not grammatically correct. It should be "very badly" or "really bad."
"Not like that" can be grammatically correct, depending on the context.
Yes, 'for free', is grammatically correct.
Yes! That is grammatically correct!
The phrase "Is you don't miss me do you" is not grammatically correct. It should be rephrased to something like "Don't you miss me?" to be correct.
The phrase "You are not knowing" is not grammatically correct. Instead, you can use "You do not know" or "You are not aware."
Yes, it is grammatically correct.
Yes, Almighty God is grammatically correct.
"That was wrong" is a grammatically correct sentence.