Lesson 28: Question nouns: なに, だれ, どれ, いつ
These are called interrogatives. They're used in order to ask wh-questions. These words can't be used as relative clause markers like english does in "The person who was fine with it". They also show up as parts of larger grammatical words.
何 なに "what"
何するの？
Whatcha doin'?
誰 だれ "who"
誰だ！
Who's there!
誰？
ho?
誰ですか
Who is it?
どれ basically "which"
どれがいいかなぁ
I wonder which one would be good.
いつ "when" (not "while" or "at the time that")
いつがいいか
When's good?
いつから君がいた、ここ？
How long have you been here?
(metaphorically, "Since when were you here?")
Japanese is a "wh-in-situ" language, which just means that it doesn't need to move interrogatives around in wh-questions like english does.