Do I have to use nan, ka, or both to ask a question?


I'm going through Genki, and for questions, it gives the example,

りゅうがくせいです (I am an international student) can be turned into a question by
adding ka to the end.

りゅうがくせいですか (Are you an international student?)

But then it starts saying that you use nan desu ka to make a question, like
in,

せんこうはなんですか (What is your major?)

So when do I use na desu ka, or just ka, or just nan, or what?




I couldn't quickly find a duplicate, but I wanted to address what Genki had to
say (my emphasis):

> It's very easy to form questions in Japanese. Basically, all you need to do
> is add _ka_ at the end of a **statement**.
>
> りゅうがくせいです。  
>  Ryuugakusee desu.  
>  (I am) an international student.
>
> りゅうがくせいです **か** 。  
>  Ryuugakusee desu ka.  
>  (Are you) an international student?

It goes on to point out that this was a "yes/no" question. In other words, you
can take a basic statement (that would be the answer to a yes/no question) and
turn it into a question by adding か at the end.

It then follows with

> Question sentences **may also** contain a "question word" like _nan_ (what).
> In this lesson, we learn how to ask, and answer, questions using the
> following **question words** : _nanji_ (what time), _nansai_ (how old),
> _nannensee_ (what year in school).
>
> せんこうは  **なん** ですか。  
>  Senkoo wa  nan desu ka  
>  What is your major?
>
> (せんこうは)    **えいご** です。  
>  (Senkoo wa)   eego desu.  
>  (My major) is English.

In other words, when you want to ask WH-question like _what?_ , you use a
question word like the above. Notice that the first question was not a WH-
question. So a WH-word was not used.

For context, you will later learn to drop certain words in casual speech, like
です and か in questions.

