形容動詞 attributively before a noun: な or conjugated だ

Question

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 of one such site.

Answer 1

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.

Answer 2

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.

Answer 3

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.