Meaning: to explain something; show emphasis
I think you might see the top-ranked "How to use んです?" blog on Google, but I think there are some mistakes in this blog.
First, " you cannot use んです to tell people how you feel " is not correct, for example:
Can you tell the difference between 1, 2 and 3?
- あついです (atsui desu)
- あついんです (atsui ndesu)
- あついんだ (atsui nda)
1 is simply stating a fact. That it/something is hot.
2 is also stating a fact, but carries a bit more emphasis as the speaker may be answering/asking a question or explaining their feelings.
3 is the same as 2, but the more casual version.
6 situations in which you should use んです
B is the reason for A
Translation: The flower has withered, because I forgot to water it.
Explanation: In this situation, んです is used to explain the previous sentence. It can be translated as “because” in English.
B is used to describe A from another angle or summarize A.
Translation: She is not married. Nor does she have any boyfriend. In other words, she is single.
Explanation: んです here can be translated as “in other words” or “that is”. It usually follows つまり(that is), 私が言いたいのは(what I want to say is) and 要するに (in short).
A is a scene or an implied situation. B is deduced from A. In this situation, the whole sentence A sometimes is skipped.
Example: (I see that the ground is wet) 雨が降ったんです。
Translation: (I see that the ground is wet) It rained.
Explanation: In this example, the whole sentence A is skipped. Only sentence B is spoken or written. And B is a conclusion based on the observation of A. んです implies that the speaker has such observation.
B is the result of A.
Translation: I look up at the cloudy sky, then I see snow fall.
Explanation: A happens and as a result B happens. んです here can be translated as “then / as a result”.
B is an order. A does not exist.
Translation: Come here quickly.
Since it is an order, normally we don’t use the polite ですform. んだ here make this sentence become an order.
To tell listeners your decision. A does not exist.
Translation: Even if my parents disagree, I’ll do it.
Explanation: んです here can be used to show resolution and judgment.
Situation 1, Situation 2, and Situation 4 can be classified as emphatic. Situation 3, Situation 5, and Situation 6 are indeed correct as the blog said.
But this blog also talked about 6 situations in which you can't use んです , I don't think there is much reference value because these sentences are too common.
In fact, these six unavailable situations can all be classified as one, and they are not "answers". I mean, none of these sentences are answering other people's questions or try to explain something, except for the sixth situation.
The sixth situation is an obvious fact and does not need to be emphasized, so んです cannot be used.
Unless you meet an animal, the animal does not look like a cat, but it is a cat, you may be able to use んです.
んです is indeed used to explain, there is this feeling in it.
I believe that if you encounter an obvious fact, but keep using んです, you will probably offend others. (I know it's a cat, why do you want to explain it to me, do you think I'm a fool?)
The reason why "あれ、財布がない" cannot be said to be "あれ、財布がないんだ" is not because it happened suddenly, but because if you say "財布がないんだ", it makes people think you know where is it. (You look like acting) (Sounds like a joke)
In conclusion, I feel that んです is used to remind the other to pay attention to what you are saying. (show emphasis)