What is the difference between 問題なのは~ and 問題は~?


> 結局の所問題なのは地面を覆っているアスファルトだ。

Why is there a なの here even though 問題 is a noun? Are there other nouns that
can have also have it, is it only restricted for 問題, or it's possible for all
nouns?

On the side note, what is the difference between 問題は and 問題なのは?




問題なのはXだ is a [cleft sentence](https://japanese.stackexchange.com/a/19208/5010)
made from Xは問題だ. So it's more emphatic.

>   * 問題はXだ。  
>  The problem is X.
>   * Xは問題だ。  
>  X is a problem.
>   * 問題なのはXだ。  
>  It is X that is the problem.  
>  What is problematic is X.
>

Note that the copula だ turns into な when it modifies の, which is technically a
(formal) noun. 問題 in 問題なのは is a simple noun (or a na-adjective) and not
special at all. You can also say 馬鹿なのは俺だ ("It is me who is an idiot"),
強い犬なのはこちらの犬です ("It is _this_ dog that is the stronger one"), and so on.




問題なのはXだ is a [cleft sentence](https://japanese.stackexchange.com/a/19208/5010)
made from Xは問題だ. So it's more emphatic.

>   * 問題はXだ。  
>  The problem is X.
>   * Xは問題だ。  
>  X is a problem.
>   * 問題なのはXだ。  
>  It is X that is the problem.  
>  What is problematic is X.
>

Note that the copula だ turns into な when it modifies の, which is technically a
(formal) noun. 問題 in 問題なのは is a simple noun (or a na-adjective) and not
special at all. You can also say 馬鹿なのは俺だ ("It is me who is an idiot"),
強い犬なのはこちらの犬です ("It is _this_ dog that is the stronger one"), and so on.

