No one knows.

Dare mo shiranai. : No one knows.
dare : who
dare mo ...nai : no one ....
shiru / shitteiru : know
(conjugation : shiranai, shirimasu, shiru, shirutoki,shireba,shire)
(conjugation : shitteinai, shitteimasu, shitteiru, shitteirutoki, shitteireba, shitteiro)

In above case, " mo " is important. You should better learn it as a phrase.

If " mo " changed to "ga", the meaning would change.

Dare ga shiranai : who, does not know : Who doesn't know?( I think everyone knows it.)

Nani mo shiranai : I know nothing.
nani : what
In this case, "nani" is an objective word.
This sentense should be learnt as one phrase, too.

Dare mo utawanai : No one sings.
Nani mo utawanai : I sing nothing.
utau : sing
(conjugation : utawanai,utaimasu, utau,utautoki,utaeba,utae)