What is this ?

nan / nani ; what
dare ; who
itsu ; when

Kore-wa hon desu. ; This is a book.
Kore-wa hon desuka? ; Is this a book?
Kore-wa nan desuka? ; What is this ?

You may understand how to make a " what " interrogative sentence.
A noun should be substituted by "nan".

are; that
sore; it
Are-wa nan desuka?; What is that?
Sore-wa nan desuka? ; What is it?

kare; he
san; Mr./Ms
dare ; who

Kare-wa Suzuki-san desu. ; He is Mr.Suzuki.
Kare-wa Suzuki-san desuka? ; Is he Mr.Suzuki?
Kare-wa dare desuka? ; Who is he?

getsuyoubi ; Monday
ni ; on

Anata-wa getsuyoubi ni gakkou e ikimasu. ; You, Monday on ,school to ,go→You go to school on Monday.
Anata-wa getsuyoubi ni gakkou e ikimasuka? ; Do you go to school on Monday?
Anata-wa itsu gakkou ni ikimasuka? ; When do you go to school?