のだ Japanese Grammar Lesson

Grammar Type: Phrase

Essential Meaning: It’s because / The reason is / It’s that


  • (Verb / い-Adjective) informal + のだ
    • E.g. はなすのだ [It’s because X speaks]
    • E.g. おおきいのだ [It’s because X is big]
  • (な-Adjective Stem / Noun) + [な / だった] + のだ
    • E.g. しずかなのだ [It’s because X is quiet]
    • E.g. 先生せんせいなのだ [It’s because X is a teacher]


  • のだ is primarily used when the speaker is explaining or asking for an explanation about information that is shared between the speaker and hearer (often info the speaker and hearer have directly heard or observed). のだ is also used imbue sentences with a natural emotional tonality (e.g. inquisitive tone, explanatory tone, surprised tone, etc.) depending on the context. Colloquially, のだ can be shortened to んだ or の (males tend to use んだ, while females tend to use の). In interrogative sentences, かい and だい are often used (かい for yes / no questions and だい for open-ended questions).
  • To understand the connotation of のだ, consider the following example: A: [After directly observing B] なにをしていますか。B: 日本語にほんご勉強べんきょうしています。[A: What are you doing? B: I’m studying Japanese.] In this scenario, Speaker A asks for an explanation about something that he directly observes Speaker B doing, and Speaker B provides an explanation. It would make sense for Speaker A to ask his question with a curious or inquisitive tone and for Speaker B to respond in an explanatory tone. But as it stands, the sentence comes across as rather flat and emotionless. のだ is used to create a more natural sense of tonality and emotion.
    • E.g. A: [After observing B] なにをしているんですか。B: 日本語にほんご勉強べんきょうしているんだ。[A: So, what are you doing? B: I’m studying Japanese.] Because Speaker A observes what Speaker B is doing, there is information symmetry regarding what Speaker B is doing. Therefore, のだ is appropriate and provides the sentence with natural emotional undertones.
  • However, if Speaker A doesn’t actually observe Speaker B doing something but only assumes that Speaker B is doing something, it’s inappropriate to use のだ to seek an explanation. In other words, the speaker and hearer need to have shared (i.e. not assumed) knowledge of the situation in order to use のだ to seek an explanation. However, Speak B can still provide the explanation using のだ.
    • E.g. While speaking on the phone: A: なにをしていますか。B: 日本語にほんご勉強べんきょうしてるんだ。[A: What are you doing? B: I’m studying Japanese.] Because the conversation occurs over the phone, there is an asymmetry in knowledge about what Speaker B is doing (i.e. Speaker B knows what he is doing, but Speaker A can only assume). When such an information asymmetry exists, のだ should not be used to seek an explanation.
  • There are also situations when のだ is used even when the information is not shared by the speaker and hearer and when the speaker is neither explaining something nor seeking an explanation. In such cases, the speaker speaks as if the information were shared with the hearer (even though it isn’t). This is similar to introducing an English sentence with “So…” or “You know?“. Essentially, the speaker introduces some topic with the expectation that the hearer will respond / react / engage in some way.
    • E.g. 今日きょうはフットボールがあるんですが一緒いっしょにいきませんか。[So…there’s a football game today. Shall we go watch it together?]
    • E.g. 先生せんせいこまっているんです。たすけてください。 [Teacher. So…I have a problem. Please help me. ]
    • E.g. 日本語にほんご文法ぶんぽうむずかしいですが面白おもしろいんですよ。[Japanese grammar is difficult, but it’s interesting, you know?]
  • You can use んだけど or んですが to connect two sentences when the 1st sentence sets the stage for or makes an implication about the content of Sentence 2. This structure takes the form Sentence 1 んだけど / んですが Sentence 2, but Sentence 2 is often omitted. To illustrate the concept, if a teenage girl sends a text to her boyfriend saying “My parents are out of town and I have the house all to myself…“, it’s clear that what she is REALLY saying is that she wants her boyfriend to come over, even though she doesn’t state this explicitly. In other words, Sentence 1 implies what the speaker wants without literally expressing that want (so Sentence 2 is unnecessary). Similarly, if I tell my friend “Hey, I have an extra ticket to the Saints game this weekend...”, I am clearly hinting at the fact that I want my friend to come with me even though I didn’t say that explicitly.
    • E.g. チケットが2まいあるんだけど。。。[I have two tickets to the game… (and I want to give one of them to you).]
    • E.g. えききたいんですが。。。[I want to go to the station… (and I’m hoping you can give me directions).]
  • This んだけど / んですが structure is not only used for asking a favor or for extending an invitation, but it can also be used to simply provide background or commentary on some topic. In such cases, Sentence 1 provides context or relevant info, and Sentence 2 provides the main message that the speaker wants to convey,
    • E.g. 昨日きのうあたらしいほんったんだけど, すごく面白おもしろい。[So I bought a new book yesterday…It’s super interesting!] In this example, the fact that the speaker bought the book yesterday is not the point. It’s simply background for the speaker’s true point, which is that the book is interesting and that he wants to talk about it further.
  • のだ is also used for drawing an assumption, interpretation, or conclusion based on perceptual evidence. For example, if a girl has red eyes and makeup dripping down her face, it’s evident that she has been crying. One might say いていたんですね。[You must have been crying, right?] Similarly, if someone’s face turns red after eating a chicken wing, it’s evident that the wing was spicy. You can express this conclusion with からいんですね。[It must be spicy.]
  • Along these same lines, のだ can be used to express discovery, doubt, or surprise. For example, consider the difference between the following two sentences: (1) あたらしいポケモンのゲームをった? [Did you buy the new Pokemon game?] vs (2) あたらしいポケモンのゲームをったの? [You bought the new Pokemon game?!] The difference is that the first sentence asks the question in a neutral manner, while the second implies emotive surprise or disbelief over the fact that the hearer bought the new Pokemon game. When のだ is shortened into の (e.g. ったの?), it is often pronounced with rising inflection to convey extra emotionality.
    • E.g. ピアスったんだ。[You bought earrings?!]
    • E.g. ハワイにったんだ。[You went to Hawaii?!]
    • E.g. 十五歳じゅうごさいなの。[You’re 15?!]
    • E.g. 妊娠にんしんしてるの。[You’re pregnant?!]
    • E.g. 彼氏かれしいたの。 [Oh, you have a boyfriend?!]
    • E.g. 鬼滅きめつやいばたんですか。[You watched Kimetsu no Yaiba?!]
    • E.g. あるけるの?[Can you walk?!] In this sentence, the speaker is expressing doubt that someone can walk (perhaps after an injury).

Example Sentences:

A: [After observing B] なにをしているんですか。B: 日本語にほんご勉強べんきょうしているんだ。[A: So, what are you doing? B: I’m studying Japanese.]

A: どうしておさけまないんですか。B: わたしはまだ17なんです。[A: Why don’t you drinking sake? B: Because I’m still 17.]

ぼく今日きょうのパーティーにけません。宿題しゅくだいがたくさんあるんです。[I can’t go to today’s party. It’s because I have a lot of homework.]

あなたと結婚けっこんしたいんです。[I want to marry you.]

WILD Examples:

This song begins with the following line: ぼくうままれた、このしまそらを、ぼくはどれくらいっているだろう [This island where I was born, I wonder how well I know its sky?] In this example, the speaker is asking a rhetorical question and のだ provides a natural tone of curiosity.
Listen to this line at 0:39: 今日きょう最期さいごかもしらないんだ [Perhaps today is my last]
Listen to this lyric at 0:41: こころからだしてきたんだ [My heart begins to overtake my body]
Listen to this line at 2:00: ずっとわすれたことがあるんだ [It’s because there were constantly times when I forgot to say it.] In this case, のだ has an explanatory connotation.

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