I'm not enough of a grammarian to give an exact answer, but I'm pretty sure that this is not a singular/plural issue. For example

"Does he know what happened?"
"He knows what happened"

>Hi everybody,
>
>I'm just curious, what would be correct from the English grammar point of view question:
>
>"Does anybody know what happened...?"
>
>"Anybody knows what happened...?"
>
>"Anybody know what happened...?"
>
>"Anybody, what's up with ...?"
>
>Or you can suggest a better form?
>
>Thanks in advance.
>
>And I'd like to know the rules of singular/plural in appliance to
>
>everybody
>nobody
>anybody
>anyone
>everyone