The んだ (んです) explanatory form is an essential part of Japanese grammar used to give explanations, seek clarifications, express reasons, and add nuance to conversations. It is commonly heard in everyday speech and is crucial for sounding natural and fluent in Japanese.
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What is the んだ / んです Explanatory Form?
The んだ (casual) / んです (polite) is a contracted form of のだ (noda) and のです (nodesu) and is used when the speaker is giving an explanation or seeking one. It is often used to justify actions, express emotions, or clarify situations.
It does not change the core meaning of a sentence but adds nuance, making the statement sound more natural and informative.
Sentence Without んだ:
どうして遅れたの？
(Dōshite okureta no?)
"Why were you late?"
ちょっと寝坊した。
(Chotto nebō shita.)
"I overslept a little."Sentence With んだ:
どうして遅れたの？
(Dōshite okureta no?)
"Why were you late?"
ちょっと寝坊したんだ。
(Chotto nebō shitan da.)
"It's because I overslept a little."The second version sounds more natural because it provides an explanation.
The ん in んだ / んです is a contraction of の, which originally came from のだ. However, while のだ still exists in writing, んだ / んです is far more common in spoken Japanese.
How to Use んだ (んです) Correctly
The structure changes slightly depending on whether the verb or adjective is affirmative, negative, in past form, etc.
| Sentence Type | Casual Form (んだ) | Polite Form (んです) |
|---|---|---|
| Noun + だ | 学生なんだ。 (Gakusei nan da.) → “I’m a student (you see).” | 学生なんです。 (Gakusei nan desu.) → “I’m a student (you see).” |
| い-adjective | 寒いんだ。 (Samui n da.) → “It’s cold (so…).” | 寒いんです。 (Samui n desu.) → “It’s cold (so…).” |
| な-adjective + だ | 元気なんだ。 (Genki nan da.) → “I’m feeling fine (so…).” | 元気なんです。 (Genki nan desu.) → “I’m feeling fine (so…).” |
| Verb (Present/Future) | 行くんだ。 (Iku n da.) → “I’m going (you see).” | 行くんです。 (Iku n desu.) → “I’m going (you see).” |
| Verb (Past) | 行ったんだ。 (Itta n da.) → “I went (you see).” | 行ったんです。 (Itta n desu.) → “I went (you see).” |
Notice that nouns and な-adjectives require な before んだ (or んです).
Common Uses of んだ (んです)
Asking for Explanations
When you want to ask why something happened, use んですか？
どうして泣いているんですか？
(Doushite naite iru n desu ka?)
"Why are you crying?"
なんでそんなに怒っているんですか？
(Nande sonna ni okotte iru n desu ka?)
"Why are you so angry?"Giving an Explanation
When explaining something in response to a question, use んだ / んです.
昨日は忙しかったんだ。
(Kinou wa isogashikatta n da.)
"I was busy yesterday."
試験があるんです。
(Shiken ga aru n desu.)
"I have an exam." (Explaining why you are studying)Emphasizing a Statement
When you want to emphasize what you’re saying, んだ adds strength to your words.
でも、どうしてもやりたいんだ！
(Demo, dōshitemo yaritain da!)
"But I really want to do it!"
医者になりたいんだ。
(Isha ni naritain da.)
"Because I want to become a doctor!"Softening a Statement or Suggestion
Using んです makes requests and suggestions sound more polite and natural.
Example 1 (Softening a request):
トイレを使ってもいいですか？
(Toire o tsukatte mo ii desu ka?)
"Can I use the restroom?"
すみません、今掃除中なんです。
(Sumimasen, ima sōji-chū nan desu.)
"Sorry, it's being cleaned right now."Example 2 (Softening an invitation):
週末、一緒に映画を見に行きませんか？
(Shūmatsu, issho ni eiga o mi ni ikimasen ka?)
"Would you like to watch a movie together this weekend?"
週末は忙しいんです。
(Shūmatsu wa isogashiin desu.)
"I’m busy this weekend (so I can’t)."んだ vs. のだ vs. の
| Form | Usage | Example |
|---|---|---|
| んだ (んです) | Conversational, natural | 日本へ行くんです。 (Nihon e iku n desu.) → “I’m going to Japan (you see).” |
| のだ (のです) | More formal, written style | 日本へ行くのです。 (Nihon e iku no desu.) → “I’m going to Japan (for this reason).” |
| の? | Casual question (mostly female speech) | 何をしているの？ (Nani o shite iru no?) → “What are you doing?” |