#older post

Definition of  だろう

Japanese: A Comprehensive Grammar, Stefan Kaiser (p. 232)

“Presumptive” means that something is based on probability. The speaker is guessing or assuming something.

～だろう・でしょう (formal) can be translated as:

I think; I suppose; I reckon; I guess

I hope (that)…

I’m afraid / I fear (that)…

perhaps; probably; maybe; likely

Simple enough, right? Except that many other structures share these same translations, which makes things murkier. This grammar is a modal structure, and modal structures in Japanese—just like in English—exist on a spectrum. Some express assumptions based on evidence; others rely purely on the speaker’s knowledge.

Japanese dictionaries define だろう as:

不確かな断定、あるいは推定の意を表す。  → It expresses an uncertain assertion or presumption.

Google Dictionary adds:

話し手（書き手）の推量によってその事柄を述べている意を表す。  → It expresses the speaker’s (or writer’s) opinion of the matter.

述べている事柄での因果関係や理由となるものを推量するのに使う。 → Used to infer a causal relationship or reason in the matter being described.

Key words: uncertain, speaker’s opinion, causal relationship.

Stefan Kaiser also notes that だろう can be translated as “should.” If “should” sounds appropriately uncertain and opinion-based to you, go for it.

Side note: Professional business writers often advise against “should” precisely because it's too vague—is it a suggestion? An order? A recommendation? That ambiguity is exactly what だろう captures.

Definition of  んだ

Japanese: A Comprehensive Grammar, Stefan Kaiser (p. 279)

“However, the force of n(o) da is rarely captured in translation.”

That’s the first thing to keep in mind. This structure will start to make sense eventually—all it takes is reading many explanations and seeing it in many contexts.

But why is んだ labeled “evaluative”? That word is a useful clue. 

To evaluate means to consider something in order to make a judgment about it—and that’s exactly what んだ does: you first take in the situation, then express your judgment about it.

Imagine you've just come back from Japan with three enormous suitcases full of anime merch. Your friends give you a look. And you say: “What?! I went to Japan, duh.” By adding “duh” and “what?!” with exasperation, you’re explaining yourself—making them understand why three suitcases are perfectly reasonable. You assessed the situation, then reacted with something that would make them understand. That’s んだ.

Wasabi lists the following uses of んだ・のだ:

Reason

Interpretation

Discovery (of non-physical information)

Summary/rewording

Preliminary remarks

Tofugu frames it as “providing an explanation” or “seeking an explanation,” noting that んです indicates the statement is based on background information or knowledge shared by the speaker and the listener. That shared knowledge element is important.

This website (no longer working) looks at this grammar from a different perspective:

“A” means a sentence / statement that does NOT have んです.

“B” means a sentence / statement that HAS んです.

   — statement B is the reason for statement A

   — B is used to describe A from another angle or summarize A

   — A is a scene or an implied situation. B is deduced from A. In this situation, the whole sentence A sometimes is skipped

   — B is the result of A

   — B is an order. A does not exist

   — To tell listeners your decision. A does not exist

The sheer number of explanations out there is itself telling—this grammar is deeply context-dependent and specific to Japanese. There’s no clean English translation. The best approach is to associate it with situations rather than definitions.

Enough theory. Let’s look at some examples.

Examples — Jujutsu Kaisen

All the examples are from Jujutsu Kaisen, episode 17, season 1.

Context from the picture above: Maki is analyzing Miwa’s tactics, and her conclusion is:

私ってよりは 呪具を払って隙を作りたいんだろう → Rather than hit me, she likely wants to knock away my cursed tool and create an opening.

ってよりは = rather than [casual: と言うよりは]

呪具 (じゅぐ) = cursed tool

払う (はらう) = to sweep; to knock aside

隙 (すき) = gap; opening

んだろう = ん + だろう = judgment + speculation based on observation

Maki is assessing her current situation and trying to reach a conclusion—judging Miwa’s moves to foresee her strategy. She’s assuming Miwa wants to knock aside her cursed tool, hence だろう. ん in front emphasizes the “interpretation” part. We could drop it, but the meaning would slightly change—the sentence would become a plain opinion with no connection to the current situation.

ん tells us that Maki has analyzed her situation thoroughly, and that’s her conclusion—a conclusion influenced by the preceding events. If Maki were just an observer who randomly wanted to make a pure guess (no supportive arguments whatsoever), then だろう alone would be ok.

In short: it’s a guess, but this guess is supported by some evidence.

One more example from Maki:

ああ？言わなくても分かんだろ！ → Do I have to spell it out?

んだろ here suggests that Maki expects Mai to already understand why she left home. They share the same history and background knowledge — so this isn’t a neutral question. Maki’s assumption is shaped by everything they’ve been through together. 

Gojo to Mei Mei, agreeing that Maki should be promoted:

僕も そう思ってるんだけどさ― 禪院 (ぜんいん) 家が邪魔してるくさいんだよねー → I think so too, but… the Zenin clan is very much in the way.

邪魔 (じゃま) = hindrance; obstacle; interference

家 is read as け here

くさい = emphasis (like 面倒くさい — it intensifies the noun before it)

さ adds a rough, informal feel. んだよね adds a nuance of seeking agreement

んだけど, according to the internet, means that our speaker is about to give us some explanation; the speaker is sort of announcing: “Focus! I will give you an explanation.” It may not happen, though. The explanation might be obvious from the context, or we don’t want to say it out loud. Wasabi refers to it as “preliminary remarks” (we want to draw our speaker’s attention to the next sentence). 

んだけど draws the listener’s attention to what comes. It’s a bit like “you see…” in English. Gojo is literally saying: “Based on what we just saw (んだ), I think so too, but…”. Dropping んだ would turn it into a plain statement with no emotional or contextual weight.

But let’s break this down:

んだ gives us this “explanatory” nuance. Gojo “explains” to us why he thinks so too (why promoting Maki is a good idea). Everyone in the room saw Maki’s performance (background information), hence adding んだ makes sense. Gojo is literally saying: “Based on what we just saw (んだ), I think so too, but...”.

Just to make it make even more sense (from above theory section):

B is the result of A = Gojo’s statement is the result of Maki’s performance.

Dropping んだ would turn Gojo’s sentence into a plain statement: “I think so too, but...” Sounds perfectly fine, doesn’t it? By “plain statement,” I mean that there are no emotions involved, no subjective feelings, and no background information.

Also, when んだ or のだ ends a sentence, it refers to some kind of reservation, which is implied or expressed.

けど is used to say “but” or in some cases “and.”

According to someone on the Internet, んだけど works a bit like “you see…” It’s a common filler phrase in English when you are explaining something to someone or encouraging them to listen and understand (Collins dic.) or it is used when you hope someone else will understand what you are saying or asking (Cambridge dic.).

僕も そう思ってるんだけどさ―  → You see, I think so too…

Doesn’t sound that bad to me.

禪院 (ぜんいん) 家が邪魔してるくさいんだよねー → Zenin clan is in the way.

邪魔 (じゃま) = hindrance; obstacle; nuisance; disturbance; interruption; interference​ [suru verb]

家 is read as け here.

Extra:

くさい :

 〜のにおいがする = smells like 〜のような様子（様子 ようす) = looks like  強調（きょうちょう) = emphasis; highlight; stress; stressed point​ 面倒くさい  → くさい emphasizes the noun 面倒.

Gojo is using it as an emphasis. They are REALLY/ VERY MUCH getting in the way. 

I’m going to leave you with this one. Think about a possible reason why Gojo used んだ in this sentence.

Mei Mei to Gojo. She’s explaining why she likes money so much.

何せ 金に換えられないんだから → After all, there’s no value to anything that can’t be traded for money.

何せ = at any rate; after all; because

換える (かえる) = to exchange; to trade

から = because.

んだから, according to the internet, is used when “both the speaker and listener know some facts, but it expresses a strong feeling on the part of the speaker that the listener, although conscious of said fact, does not fully appreciate its implications.”

んだから is used when both the speaker and listener know some facts, but the speaker feels the listener doesn’t fully appreciate the implications. Mei Mei presumes Gojo already knows her personality—she’s reaffirming something he’s aware of and nudging him to agree with her reasoning. 

However, you can just assume that んだ is a contextual explanation and から works like good old “because.”

One more example is here.

Hedgehog-Japanese.com explains this grammar in the following way:

The “A nodakara B” [or ndakara] is used to state a reason. It is the same as ”A kara B" in terms of being used to state a reason, but there are restrictions on what can be placed in “A” and “B.” The “A” contains a reaffirmation of something that the listener already knows, and the ”B” contains advice, commands, requests, etc., or the speaker’s opinion to persuade the listener, or the speaker’s opinion with which he or she wants the listener to agree or concur.

Following Hedgehog’s definition, we can assume that our sentence with んだから contains a reaffirmation of something that the listener already knows = Gojo knows Mei Mei and is quite aware of her personality and her love for money. The thing Mei Mei said before that sentence (”There’s no value to...”) is Mei Mei opinion (explanation as well) and she wants Gojo to “agree” with her opinion. I think it makes sense because Mei Mei’s love for money is unusual and I feel like she often needs to explain why she loves money so much.

Nobara to Momo: 

こちとら―ぶん投げられ慣れてんだよ → I’m used to getting thrown around, you know.

こちとら = こちらとしては = casual “I” / “as for me”

ぶん投げる (ぶんなげる) = to throw (violently)

慣れる (なれる) = to get used to; to grow accustomed to

Momo told Nobara that female sorcerers must be strong and beautiful; otherwise, they would be underestimated and undervalued. Nobara is a badass girl, and she is used to being knocked around (knocked around = underestimated and undervalued). Momo was also blasting Nobara relentlessly. But that didn’t discourage Nobara.

んだ here is an explanation. Nobara wants to explain why Momo’s line of reasoning makes no sense to her. English fillers like “you see” or “you know” work well here. 

“I’m used to getting knocked around” and the implied part is: whether it’s you or someone else, I’m used to it, just so you know.

The conversation continues:

男がどうとか 女がどうとか 知ったこっちゃねえんだよ! てめえらだけで 勝手にやってろ → I don’t give a damn about how men are or how women are! Sort it out yourselves.

知ったこっちゃねえんだよ = I don’t give a damn about…

知ったこっちゃねえ = 知ったこっちゃない = 知ったことではない

ねえ is a rough way to say ない

どうとか ＝どう ＋とか (Nobara is saying something like, “how X is and all that” (“How men are [男がどう] and all that [とか])

Again, んだ as explanation—Nobara gives Momo yet another reason why her logic doesn’t land. “You need to understand… whatever you say doesn’t impress me because I don’t care about those rules.”

Maki to Mai, who is standing still and refusing to move:

どうしたんだよ？早くしろよ → What’s wrong? Hurry up.

んだ = seeking explanation in this particular situation

早くしろよ = hurry up

んだ here is seeking an explanation. Maki could ask どうしたの？ but that would sound plain—a neutral question with no connection to the situation. どうしたんだよ implies: “I can see something is going on—give me the full picture.”

Mid-post summary

んだ is:

subjective — it carries the speaker’s feelings and observations

context-dependent — it heavily relies on what came before

emphatic — it adds weight to your sentence

shared-knowledge-based — both speaker and listener need to be on the same page; if one of you doesn’t know the background, it can lead to confusion

Real-life examples of んだ

From a an article

「進路指導で地元の国公大学を強く薦められます。できれば東京の私立大学にいきたいんですが、先生の言うことを聞いたほうがいいですか？」。受験の意思決定を考える → During career counseling, I'm being strongly recommended a local public university. I want to go to a private university in Tokyo, though, so should I listen to what my teacher says?

When you add ん to a verb, you aren’t just stating a cold fact; you are actively explaining a situation, giving background context, or revealing the hidden "why" behind your emotions.

Why did the student use ん here? Because of shared information.

The student already established the background: The teacher is pressuring them to stay local. That information is now sitting on the table, shared between the writer and the reader.

By adding ん to the next sentence (ikitai ndesu), the student connects their desire to that shared background. It's like saying:"Given the fact that my teacher is pressuring me... here is the explanation for my dilemma: I actually want to go to Tokyo."

Without the ん, the text would feel robotic—just two unrelated facts slapped next to each other. With the ん, it becomes a cohesive emotional struggle.

Compare:

Without ん: "I want to go to Tokyo. Should I listen to my teacher?" (Sounds a bit abrupt and disjointed).

With ん and が: "I want to go to Tokyo, though... so should I listen to my teacher?" (Signals a complication and hints that you're stuck).

From actual conversations with my Japanese friends:

いつまで続くんだろう。。。 → I wonder how long it will last… (about the state of emergency in Tokyo)

んだろう because it refers to shared observations—we both knew what was happening at the time.

今でも実際会ってレッスンを受けたいって思うんだろうね → I guess some people still want to meet in person for lessons. (about online teaching)

A guess based on observations, with shared background knowledge — んだろう fits perfectly.

楽しんで勉強してたつもりなんだけど。 → I thought I was studying and enjoying it, but…

Here んだけど is used to complain a little. She didn’t say anything else after this sentence. The implication is left unspoken, which is itself very んだ.

Links to check out

～んだ (のです) grammar – part 2

～んだろう grammar

～んだから grammar