Why is だ used with question words such as 何だ?


Since だ is declarative, how would it work in a question? Why would i say これ何だ
over これなに ? How does a declarative in a question work?




There is often a difference in nuance between information questions asked with
「だ」 and those asked without.

> **Without 「だ」** :
>
> 「これ（は）なんですか？」、「これ（は）なに？」、「これ（は）なんなの？」, etc.

These are more versatile in usage because they are neutral in nuance. They
sound neutral unless you express some kind of emotion, such as anger,
surprise, etc. in your tone of voice and/or by adding additional words or
phrases that can express a particular kind of emotion.

For that versatility, those questions can be asked among all types of
individuals and in all types of situations where you want to find out what
something is.

> **With 「だ」** :
>
> 「これ（は）なん **だ** ？」

This information question using 「だ」 is often, if not always, used in different
situations and with different people. Examples would include:

1) You are talking to yourself, either silently or out loud. Using 「だ」 is
quite common when talking to yourself.

2) You are talking to your child, student or subordinate in a scolding manner,
demanding a good explanation about a thing or matter.

Note that the 「だ」 questions will not be asked the other way around in the
relationship. That is by a child to his parent, for instance. Use the examples
in the first group above to ask people in higher status than you information
questions.

3) This partly overlaps with 1), but when asking yourself **_profound_**
and/or **_philosophical_** questions, you would often use 「だ」. Those would be
questions like 「人生{じんせい}とはなん **だ** ？」("What is life?") and 「SEとはなん **だ**
？」("What is StackExchange?") .

