usage - When do you use the "な" version of the copula?

As I understand it, the copula "だ" turns into "な" in some situations. The most obvious one is after な-adjectives, but it often appears after nouns too, like in this sentence: "スミスさんは、将来日本の会社で働くのが夢なんだって".

Are there any rules for when だ turns into な? It seems to me that if the copula shows up at the end of a sentence it's "だ" and elsewhere it's "な", but I have no idea if it's that simple.

Answer

when does the copula become "な"?

Simply, だ turns into な when it modifies the following noun, which is の/ん in this case. な is the attributive form of the copula だ. So-called na-adjectives and ordinary nouns are technically both nouns, so they share the similar grammar pattern.

This の/ん is a tiny special noun commonly referred to as explanatory-no. This の/ん is a big topic in Japanese, so please read these first:

So, 夢なのだ/夢なんだ has an added nuance as compared to simple 夢だ. "スミスさんは将来日本の会社で働くのが夢なんだって" may be used in the following situations:

1. When this sentence works as an explanation/clarification of the previous context.
   

   
   

   スミスさんは毎日、日本語を一生懸命勉強してるよ。将来日本の会社で働くのが夢なんだって。

   
   

   
   


2. When this sentence works as a preliminary remark (i.e., background information).
   
   

   
   

   スミスさんは将来日本の会社で働くのが夢なんだって。お勧めの会社を知ってたら紹介してくれない？

   
   

   
   


3. When you just realized this fact.
   

   
   

   ねえ、今本人から聞いたんだけど、スミスさんは将来日本の会社で働くのが夢なんだって!