So, I thought I understood the use of the 連体形 of だ, な, after a "な-adjective"
when used before a noun, however I'm a bit confused. On IMABI and a number of
other sites, I've found the suggestion to use the conjugation of だ when using
a "な-adjective" attributively before a noun. Here is an example sentence:

> 大切だった人だ。  
>  (was an important person)

So, why/when wouldn't you just use:`大切な人だった。`?

Here is a [link](https://www.imabi.net/adjectivesii.htm) of one such site.
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2018-08-08T16:46:58.127
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question
adjectives copula
形容動詞 attributively before a noun: な or conjugated だ
Great question! There is a slight difference between 大切な人だった and 大切だった人.

`大切な人だった` means "(somebody) Was an important person" just like you would
expect.

`大切だった人` means "A person that was important". Again, this is a similar
meaning, but you use each form in different situations. For example:

> エイブラハム・リンカーンは重要な人物だった。
>
> Abraham Lincoln was an important person.

vs

> アメリカの歴史｛れきし｝において重要だったエイブラハム・リンカーンは、内乱｛ないらん｝を終｛お｝わらせた。
>
> Abraham Lincoln, an important figure in American history, ended the Civil
> War.

Using a different adjective, 簡単｛かんたん｝: simple

> この問題｛もんだい｝を解｛と｝くのは簡単｛かんたん｝だった。
>
> Solving this problem was simple.

vs

> 簡単｛かんたん｝だった問題｛もんだい｝からは、なにも学｛まな｝びませんでした。
>
> I learned nothing from the problems that were simple.
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When someone was once important to you, you can refer to it as 大切な人だった. Now
what if you are asked in the moment what kind of person s/he is? Then, you say
大切だった人だ.
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2018-08-08T23:47:03.830
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> 大切だった人だ

That would mean "he is (even now) a person that was important". You could say,
for example, that even now you appreciate how important he used to be.

> 大切な人だった

You literally say that he was an important person.

The difference between them is mostly the nuance. They mean the same thing.
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