In short

〜んです (n desu) does not change what a sentence says - it changes why you are saying it. It frames the sentence as an explanation of the situation you and the listener are both in: “the thing is…”, “that’s because…”, “so that’s why…”. Attach it to the plain form, and remember that nouns and な-adjectives need a な in between (学生なんです gakusei na n desu).

The rule

Step 1 - Build the plain form, then attach ん + です.

The base is always the plain form from lesson 17 - dictionary, ない, た or なかった. です stays on the outside and carries the politeness, so the sentence is still polite even though the verb inside it is plain.

• 行く iku -> 行くんです iku n desu
• 行かない ikanai -> 行かないんです ikanai n desu
• 行った itta -> 行ったんです itta n desu
• 高い takai -> 高いんです takai n desu

Step 2 - Insert な after a noun or a な-adjective.

Verbs and い-adjectives attach directly. Nouns and な-adjectives cannot sit in front of ん on their own: the plain copula だ turns into な here.

• 学生だ -> 学生なんです gakusei na n desu (it’s that I’m a student)
• 静かだ -> 静かなんです shizuka na n desu (the thing is, it’s quiet)

This な is the same な that appears before ので and のに (学生なので, 静かなのに) - it is the noun-modifying shape of だ, because ん is really a noun (の). That is also why the written form is 〜のだ.

Step 3 - Know the four jobs it does.

1. Asking for an explanation. You see something and want the story behind it: どうしたんですか。 (Dōshita n desu ka.) What’s the matter? A plain どうしましたか asks for a report; 〜んですか asks the listener to make sense of what you can see.
2. Giving an explanation. Answering that question, or accounting for your own behaviour: お腹が痛いんです。 (Onaka ga itai n desu.) The thing is, my stomach hurts. Without ん it is a bare medical fact; with ん it explains why you look unwell or why you are leaving.
3. Setting up background before a request or an invitation. This is the highest-frequency use in daily life, and it usually ends in 〜んですが with the sentence left hanging (see Step 4).
4. Expressing realization. You have just fitted a new fact into the picture: なるほど、それで遅れたんですね。 (Naruhodo, sore de okureta n desu ne.) I see, so that’s why you were late.

Step 4 - 〜んですが: the softening preface.

〜んですが states your situation, then stops. The が is the “but/and” of a sentence you deliberately do not finish - the listener supplies the rest. It is the standard polite way into a request, a complaint or an invitation, because it lets the other person offer before you have to ask:

• すみません、道が分からないんですが…… (Sumimasen, michi ga wakaranai n desu ga) Excuse me, the thing is I can’t find my way…
• 実は、来週日本へ行くんですが…… (Jitsu wa, raishū Nihon e iku n desu ga) Actually, I’m going to Japan next week, and…

実は (jitsu wa, actually / the truth is) is its natural partner: 実は opens the background, 〜んですが closes it and hands over the turn.

Step 5 - どうして〜んですか is answered with 〜からです.

A why-question normally takes ん, because you are asking for an explanation by definition. The natural short answer is a reason clause plus です:

• どうして昨日来なかったんですか。 (Dōshite kinō konakatta n desu ka.) - 熱があったからです。 (Netsu ga atta kara desu.)

Note the shape: plain form + からです, not ×〜ましたからです. The から and ので clauses themselves are the subject of lesson 21.

Step 6 - Do not sprinkle it everywhere. Leave ん out when the sentence is a neutral fact, a self-introduction, or the first mention of something with no shared situation to explain. 私は田中です。会社員です。 is correct; ×私は田中なんです sounds as if you are correcting a misunderstanding.

Register. Casual speech uses 〜んだ (n da) for statements and 〜の for both: 〜の with rising pitch is a casual question (どこへ行くの？), with falling pitch a casual statement (お腹がすいたの。), though the falling 〜の statement has a feminine ring - men normally use 〜んだ for it. Writing and formal speech use 〜のだ／〜のである, which belongs to Written and formal register.

Forms & patterns

Attachment by word class - the な is the whole trick:

Only the non-past affirmative of nouns and な-adjectives takes な. Once you are in the past (だった) or the negative (じゃない), the form already ends like a plain predicate and ん attaches directly: 学生じゃないんです gakusei ja nai n desu.

Register ladder - same grammar, four coats of paint:

With ん or without - what changes:

Examples

Common mistakes

• 学生んです。 (Gakusei n desu.) -> 学生なんです。 (Gakusei na n desu.) - a noun cannot touch ん directly; the plain copula だ appears as な in front of it.
• 静かだんです。 (Shizuka da n desu.) -> 静かなんです。 (Shizuka na n desu.) - な-adjectives behave like nouns here: だ becomes な, it does not survive as だ.
• 高いなんです。 (Takai na n desu.) -> 高いんです。 (Takai n desu.) - い-adjectives and verbs attach directly. The な belongs only to nouns and な-adjectives.
• 行きますんです。 (Ikimasu n desu.) -> 行くんです。 (Iku n desu.) - ん follows the plain form. Politeness is carried once, by the です on the outside; ます inside is a double marking.
• はじめまして。田中なんです。 (Hajimemashite. Tanaka na n desu.) -> はじめまして。田中です。 (Hajimemashite. Tanaka desu.) - a first mention explains nothing, so ん is wrong; it would imply you are clearing up a mistaken belief about who you are.

Tips

• Translate it as “the thing is…” or “so that’s why…”, never as “it is”. If neither English phrase fits your sentence, you probably do not want ん. That single test prevents most over-use.
• 〜んですか can sound nosy or accusatory. Because it demands an account of what the listener is doing, どこへ行くんですか can land as “and where do you think you’re going?” With strangers and superiors the bare question - どこへ行きますか - is the safer choice; keep 〜んですか for situations that genuinely call for an explanation.
• The hanging 〜んですが is a politeness strategy, not an unfinished sentence. Stopping after が is exactly the point: it invites the listener to offer help rather than forcing you to demand it. Say the が, then pause.
• ん is a contraction of の, so the grammar it takes is noun grammar. That is why な appears before it, and why the written form is 〜のだ. Remembering “ん = の = noun” makes the whole paradigm fall out instead of needing memorization.