When should you use Nani / nan, as opposed to just finishing with desu ka?

Question

Inside of my genki 1 workbook, we're supposed to present questions to the answers Kimura-san gives us.

For example :

1. You: _________

   Kimura-san: よねんせいです

In this case, the answer key states that the answer is: "なんねんせいですか"

The part that confuses me is for the 2nd question:

2. You: __________

   Kimura-san: れきしです

Here, I put "なんせんこうですか", but the answer key says that the correct answer is "せんこうはなんですか"

How does one know which arrangement to use? If the structure used in the 2nd works, then does that mean that "ねんせいはなんですか" is also a viable answer for the first question? Or could we also say "なんせんこうですか" for the 2nd question?

Thank you!

Answer 1

It's more to do what kind of expressions are available rather than logic.
The both set of dialogue are based on the same structure:

～は なん です か。

A:　（がくねんは）なん ねん せい　です　か。
B:　よ ねん せい　です。

A:　せんこう は　なん です か。
B:　れきし　です。

For things that have a unit to count (ex. ねんせい/school year), we incorporate it into our question, and because of the use of ねんせい, we feel it's redundant to say がくねんは. This is about customary. It's too apparent for us when we hear なんねんせいですか, it's talking about がくねん.

On the other hand, the study subject is not applicable for any counting unit, so it's simply asked by being represented by なん.

Why it's なん instead of なに is said that it's for the easiness of pronouncing it, so, for learners, I think it's also something just have to know rather than something you can come up with by logically thinking about it.