だ　and か, Can they be used together?


One of my textbook says: だ and か can not be used together, like:

Example: これは本か。(Is this a book?)

You can NOT say: これは本だか。

The textbook also says:

疑問詞（ぎもんし）（どれ、どちら、どなた、どこ、だれ、いつ etc) When used in a sentence with か, you should
not use だ.

Another example: どの人が李先生か知っていますか。

You can NOT say: どの人が李先生だか知っていますか。

However, when I surf the Japanese websites, I noticed sentences like this:

この人だれ **だ** か分かりますか？ Here, だ and か is used together.

Isn't it supposed to be この人だれか分かりますか？

Does that mean the textbook is not accurate?




You can write だか in an embedded clause (usually containing a question word
such as どの or だれ) which is a complement of a verb such as 分かる. In a main
clause, you must omit だ before か.

So I think you **can** say:

> どの人が李先生だか知っていますか？

