What's the purpose of な in なのに?


I don't understand the role of "な" when used before "のに" as in these two
sentences (with given translations):

> 朝目がさめて初めて、外が雪 **な** のに気がついた。

He noticed that there was snow outside only after he woke up in the morning.

> せっかくの日曜日 **な** のに働いた。

Although Sunday is precious (to me), I worked (all day long).

I'm similarly confused by the なので pattern, but I assume the principle is the
same there. My best guesses so far are that:

  1. There's some relation to な-adjectives, but I don't see how 雪 or 日曜日 would be acting as adjectives here.
  2. The な has essentially the same role as だ (i.e. a "copula"?), but you can't use だ in the middle of a sentence like that.




You are on the right track. `の` is a formal noun with a general meaning such
as `the case`, `the fact`, or `the situation`. The sequence `...な` is an
appositive clause modifying the formal noun `の`.

> 彼は学生だ  
>  'He is a student.'
>
> [彼は学生な]のだ  
>  'It is the case [that he is a student].'  
>  The appositive clause `彼は学生な` 'that he is a student' modifies the formal
> noun `の` 'the case'.

As you suspect, `な` is the form of copula that is used exclusively when it is
used in a clause that modifies a noun (i.e., relative clause or appositive
clause).

> 彼は静かだ  
>  'He is quiet.'
>
> [静かな]人  
>  'person [who is quiet]' (relative clause with na-adjective as a predicate)

Na-adjectives are actually very close to nouns, and depending on the
theoretical framework, they can be actually considered a subclass of noun. You
can usually observe this `な` form used together with na-adjevtives, but with
nouns, you don't usually see it except for a number of fixed expressions like
the one you see here because with nouns, you have the option of using the
genitive case particle `の`, and that is preferred.

> 彼は学生だ  
>  'He is a student.'
>
> × [学生な]人  
>  'person [who is a student]' (relative clause with a noun as a predicate;
> usually considered ungrammatical to use `な`)
>
> 学生の人  
>  (Uses genitive case particle)




You are on the right track. `の` is a formal noun with a general meaning such
as `the case`, `the fact`, or `the situation`. The sequence `...な` is an
appositive clause modifying the formal noun `の`.

> 彼は学生だ  
>  'He is a student.'
>
> [彼は学生な]のだ  
>  'It is the case [that he is a student].'  
>  The appositive clause `彼は学生な` 'that he is a student' modifies the formal
> noun `の` 'the case'.

As you suspect, `な` is the form of copula that is used exclusively when it is
used in a clause that modifies a noun (i.e., relative clause or appositive
clause).

> 彼は静かだ  
>  'He is quiet.'
>
> [静かな]人  
>  'person [who is quiet]' (relative clause with na-adjective as a predicate)

Na-adjectives are actually very close to nouns, and depending on the
theoretical framework, they can be actually considered a subclass of noun. You
can usually observe this `な` form used together with na-adjevtives, but with
nouns, you don't usually see it except for a number of fixed expressions like
the one you see here because with nouns, you have the option of using the
genitive case particle `の`, and that is preferred.

> 彼は学生だ  
>  'He is a student.'
>
> × [学生な]人  
>  'person [who is a student]' (relative clause with a noun as a predicate;
> usually considered ungrammatical to use `な`)
>
> 学生の人  
>  (Uses genitive case particle)

