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I know what んです and its inflections mean, however, I've seen some set expressions such as そうなんです use exclusive なんです, I've never seen そうんです.
Do both なんです and んです mean the same? If so, is there some sort of rule governing when to use one or the other?
Could I replace のなんです with んです in the following sentence?

この包丁...有名な刀匠が作ったものなんです。

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I think you have a few things mixed up.

Let's start with んです. This is not just one thing. It's two: ん+です where ん is just short for the nominalizer の. Generally, this may be untranslatable when you try to bring it back into English. But, in Japanese it's serving to give some kind of explanation for why you did something, think something, or why something happened. As a nominalizer, nouns, adjectives, etc have to be properly inflected before it.

If you wanted to say, "It's a dog", then you could say

犬です

That would be formal. Informally, it would be:

犬だ

But let's assume that someone questioned you about something and you're trying to explain yourself. Then you would possibly use the んです construction and say

犬なんです
It's a dog.

Here な is the form that the copula だ takes before a noun it's modifying.

When you're saying something like

そうなんです

This breaks into three parts:

そうな + ん + です

where そうな is the form that そうだ takes as a relative clause. Put another way

そうなんです = そうなのです

A more literal translation of this would be

It's the case that it is thus.

そうんです is just ungrammatical.

なんです and んです are not at all the same thing. Neither are really complete sentences. The first parses as

 な + ん + です
[copula] + nominalizer + copula(formal)

The second parses just as

んです
nominalizer + copula(formal)

which doesn't make a whole lot of sense on it's own like that.

Now consider your example

この包丁...有名な刀匠が作ったものなんです。

This is really a sentence followed by the nominalizer の acting as providing an informal reason for something about to be said or already discussed.

The sentence is

この包丁は有名な刀匠が作ったものです

To nominalize it you can think of it going through the following steps

この包丁は有名な刀匠が作ったものです + の + です

change the first formal です to its informal form だ

この包丁は有名な刀匠が作ったものだ + の + です

change だ to the form it must take in a relative clause, な

この包丁は有名な刀匠が作ったものな + の + です

contract の to ん

この包丁は有名な刀匠が作ったものな + ん + です

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  • 2
    Such a fantastic answer, thank you so much!
    – Viktor
    Aug 14 '17 at 2:19
  • 1
    very helpful explanation! Nov 17 '20 at 10:54
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Also most adjectives that end with い end in ん Example: 優しいんです、作っていきたいんです

Everything else (non い) なんです

It seems there are exceptions like 得意(とくい) but that is still なん so it takes some getting used to

静かなんです。

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Explained as simply as possible, you need な after a noun or a な adjective to connect it to んです. い adjective or verb + んです is the same as な adjective or noun + なんです.

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