Here is a three part series of the many ways you can express ～んです (~ndesu). Careful, there are nuances in which this can convey drastically different meanings.

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～んです can be difficult for Japanese learners to use correctly. Grammatically, it&rsquo;s easy to use it: ～んです simply just comes after the dictionary form of a word, and it always comes at the end of a sentence or phrase. So what makes it so problematic for learners? The answer is that ～んです has a social function, so the trick is knowing when it&rsquo;s appropriate to use it.
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But first, here are common variations depending on politeness level and the preceding word.
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<ul>
<li>Casual conversation, following a verb &amp; い adjective: ～んだ or ～のだ</li>
<li>Casual conversation, following a noun &amp; な adjective: ～なんだ</li>
<li>Casual conversation, following anything: ～の／〜ん</li>
<li>Polite conversation, following a verb &amp; い adjective: ～んです or ～のです</li>
<li>Polite conversation, following a noun &amp; な adjective: ～なんですor なのです</li>
</ul>
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There are eight main ways that ～んです is used.
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<h3>1. 説明：Explanation</h3>
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The first person will end a question with んです to request an explanation from their partner, and the second person answers with んです to provide an explanation.
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Example 1
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<ul>
<li>Person A:「どうして遅刻したんですか？」 
<ul>
<li>&ldquo;Why were you late?&rdquo;</li>
</ul>
</li>
<li>Person B:「すみません。込んでいたんです。」 
<ul>
<li>&ldquo;I&rsquo;m sorry. It was crowded.&rdquo;</li>
</ul>
</li>
</ul>
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Example 2
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<ul>
<li>Person A :「レストランに行こ？」 
<ul>
<li>&ldquo;Want to go to a restaurant?</li>
</ul>
</li>
<li>Person B :「 レストランに行きたいってことは、お腹が空いてるの？」 
<ul>
<li>&ldquo;Since you said you want to go to a restaurant, I take it you&rsquo;re hungry?&rdquo;</li>
</ul>
</li>
</ul>
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<h3>2. 納得：Realization / Understanding</h3>
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んです can be used at the end of a statement to express that the speaker just discovered or realized something, and might even convey the feeling of surprise.
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Example 3
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<ul>
<li>「どうやったらこの機械動くのかな？あー。ここを押せばいいんだ。」</li>
<li>&ldquo;I wonder how this machine works&hellip; Ah! If I press here, it works.&rdquo;</li>
</ul>
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Example 4
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<ul>
<li>Person A :（英語を教えているとき）「ここは"it"じゃなくて"this"だよ！」 
<ul>
<li>(When teaching English) &ldquo;It shouldn&rsquo;t be &lsquo;it&rsquo; here. It&rsquo;s &lsquo;this&rsquo;!&rdquo;</li>
</ul>
</li>
<li>Person B :「そうなんですね！」 
<ul>
<li>&ldquo;I see!&rdquo;</li>
</ul>
</li>
</ul>
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Related to this, 「そうなんですね」and「そうなんですか」are polite phrases that listeners use to show that what they&rsquo;ve just heard is new and interesting to them. The English meaning is similar to, i.e. &ldquo;Is that so?&rdquo;. The listeners are mostly showing that they understand and are paying attention, but there&rsquo;s a small nuance of being surprised or having just learned something.
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Example 5
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<ul>
<li>Person A :（たくさん食べている） 
<ul>
<li>(Person A is eating a lot)</li>
</ul>
</li>
<li>Person B :「たくさん食べるんですね！」 
<ul>
<li>&ldquo;You&rsquo;re sure eating a lot!&rdquo;</li>
</ul>
</li>
</ul>
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For this particular social function of んです, it&rsquo;s often paired with repeating back what the person&rsquo;s partner has just said. An important part of Japanese culture is &ldquo;listening out loud.&rdquo; Listeners make brief verbal utterances called 相槌 （あいづち） to show the other person that they&rsquo;re listening attentively. Common 相槌 are うん, はい, and そうですか, which are similar to when English speakers say, &ldquo;Right,&rdquo; &ldquo;Really?&rdquo; or &ldquo;Is that so?&rdquo; to show they&rsquo;re listening.
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相槌 can also take the form of the listener repeating back what they just heard in the form of a question, with んですか added at the end. Because it&rsquo;s technically a question, it might seem like the listener is seeking confirmation or expressing surprise. However, the listener is simply showing that they&rsquo;re paying attention and comprehend what was spoken.
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Example 6
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<ul>
<li>Person A :「おれは今度のX JAPANのコンサートに行くよ！」 
<ul>
<li>&ldquo;I&rsquo;m going to the next X JAPAN concert!&rdquo;</li>
</ul>
</li>
<li>Person B :「えっ！先輩行くんですか？！じゃあ私も行こうかな！」 
<ul>
<li>&ldquo;Wow! You&rsquo;re going? Then maybe I&rsquo;ll go, too! 
<ul>
<li>んです can also be added at the end of a statement that paraphrases what the other person has said. Again, it&rsquo;s signaling that you understood.</li>
</ul>
</li>
</ul>
</li>
</ul>
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Example 7
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<ul>
<li>(During a break at work)</li>
<li>Person A : 「Bさん休憩時間ですよ！いっしょにお昼ご飯食べよ！」 
<ul>
<li>&ldquo;B-san, it&rsquo;s break time! Let&rsquo;s go get lunch.&rdquo;</li>
</ul>
</li>
<li>Person B : 「ごめん。これを早く終わらせないといけないからもう少しするよ。」 
<ul>
<li>&ldquo;Sorry. I have to finish this, so I&rsquo;m going to do a little more.&rdquo;</li>
</ul>
</li>
<li>Person A : 「えっ働くんだ！？うん。分かった。」 
<ul>
<li>&ldquo;What, you&rsquo;re working!? Okay, I got it.&rdquo;</li>
</ul>
</li>
</ul>
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Example 8
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<ul>
<li>Person A : 「じゃあまたね！」
<ul>
<li>&ldquo;See you later!&rdquo;</li>
</ul>
</li>
<li>Person B : 「え〜もう行くんだあ、、、。寂しいなあ。」
<ul>
<li>&ldquo;You&rsquo;re leaving already&hellip;! I&rsquo;ll be lonely.&rdquo;</li>
</ul>
</li>
</ul>
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Example 9
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<ul>
<li>Person A : 「白夜行もう一回観よ！」
<ul>
<li>&ldquo;I&rsquo;m going to watch White Night Train one more time!&rdquo;</li>
</ul>
</li>
<li>Person B : 「また観るんだ。。。」 
<ul>
<li>&ldquo;You&rsquo;re watching it again&hellip;?!&rdquo;</li>
</ul>
</li>
</ul>
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<h3>3. 先触れ、前置き：To preface or forewarn</h3>
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んです can be used at the end of an opening statement to indicate that you&rsquo;re going to immediately follow it with an announcement, news, an explanation, or the main point, etc. This use of んです is usually followed by the particle が . When んですが occurs in the middle of a sentence, it&rsquo;s a polite way to prepare the listener for what you really want to say or ask. (In casual speech, it takes the form of んだけど.）
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Example 10
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<ul>
<li>「〜さんはとても日本語がお上手なんですが、どこで勉強しましたか？」</li>
<li>&ldquo;~san, you&rsquo;re quite good at Japanese, so where did you study?&rdquo;</li>
</ul>
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Example 11
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<ul>
<li>Student:「先生、ちょっとお話があるんですが、今よろしいですか？」 
<ul>
<li>&ldquo;Sensei, I have something to tell you. Is this a good time?&rdquo;</li>
</ul>
</li>
<li>Teacher:「なんですか？」 
<ul>
<li>&ldquo;What is it?&rdquo;</li>
</ul>
</li>
<li>Student:「実は来年帰国することに決めました。」 
<ul>
<li>&ldquo;The thing is, I&rsquo;ve decided to return to my own country next year.&rdquo;</li>
</ul>
</li>
</ul>
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Example 12
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<ul>
<li>(Before asking a question)</li>
<li>「ちょっと個人的なことで、恐縮なんですが、今ご相談させていただいてもよろしいでしょうか？」</li>
<li>&ldquo;This is somewhat personal so I apologize in advance, but is it alright to discuss something with you now?&rdquo;.</li>
</ul>
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Here is&nbsp;<a href="https://www.italki.com/article/1186/Language-As-A-Window-Into-Culture:-Mastering-The-Nuances-Of-%E3%82%93%E3%81%A7%E3%81%99---Part-2">part 2</a> and&nbsp;<a href="https://www.italki.com/article/1188/Language-As-A-Window-Into-Culture:-Mastering-The-Nuances-Of-%E3%82%93%E3%81%A7%E3%81%99---Part-3">part 3</a> of this article topic on the nuances of んです (~ndesu)!
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<a href="https://unsplash.com/photos/_UIN-pFfJ7c">Hero image</a>&nbsp;by&nbsp;<a href="https://unsplash.com/@boontohhgraphy">Sorasak</a>&nbsp;on&nbsp;<a href="https://unsplash.com/">Unsplash</a>