Usage of の before です


For example

> この中でどれが一番難しいのですか。
>
> 数学が一番難しいのです

Is the の here used as nominalizer meaning "one" or is it used as
explanatory/emphasizing のです?

I assume the sentences translate to "Amongst these (courses) which is the most
difficult **one**? Math is the most difficult **one**."

Therefore I think the の here is a nominalizer, but I rememberer reading that
nominalizing の cannot be used before です and we should use こと instead. For
example:

> 私の趣味は映画を見るのです。x
>
> 私の趣味は映画を見ることです。√




Usage of の devides into (1) pronoun and (2) nominalizer.

  1. 難しいのと簡単なやつ、どっちがいい？ Which do you like, difficult one and easy one?
  2. それが難しいのは知っている I know that it's difficult.

And, when the nominalizer appears in the position of the predecate of the
sentence, we call it "explanatory/emphasizing の".

Speaking of your examples, you can regard この中でどれが一番難しいのですか and 数学が一番難しいのです as
either pronoun or explanatory の.

As you say, you can't use nominalizer の in the position of the predecate
unless it's explanatory/emphasizing の. In other words, の can be used before です
when it's a pronoun or explanatory/emphasizing の.

  * そのメガネは映画を見るのです Those glasses are one to watch movies
  * 何するんですか？ → 映画を見るんです




Usage of の devides into (1) pronoun and (2) nominalizer.

  1. 難しいのと簡単なやつ、どっちがいい？ Which do you like, difficult one and easy one?
  2. それが難しいのは知っている I know that it's difficult.

And, when the nominalizer appears in the position of the predecate of the
sentence, we call it "explanatory/emphasizing の".

Speaking of your examples, you can regard この中でどれが一番難しいのですか and 数学が一番難しいのです as
either pronoun or explanatory の.

As you say, you can't use nominalizer の in the position of the predecate
unless it's explanatory/emphasizing の. In other words, の can be used before です
when it's a pronoun or explanatory/emphasizing の.

  * そのメガネは映画を見るのです Those glasses are one to watch movies
  * 何するんですか？ → 映画を見るんです

