Specifically, in the expression

> 好き **なんだ** (I love you)

why not just say

> 好きだ

or

> 好きです

?
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2011-07-11T15:28:14.053
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2021-10-30T21:27:55.650
2020-12-24T17:09:14.750
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nuances contractions formal-nouns no-da
What connotation does なんだ add?
なんだ is a pattern that is sometimes called the "extended predicate". The exact
best way to express this in English is subject to debate.

Usually the usage follows a pattern of explanation of some question that
either has been asked explicitly or could be asked implicitly. For example, if
A-san is telling B-san that s/he wants to go with B-san to Tokyo, "好きなんだ"
provides an explanation of why A-san wants to go.

My personal choice of translating this is to use the form "It's that..." or
the longer form "It is the case that...". Although this is not a precise
translation, but in some contexts it provides a workable distinction from the
translation of the more direct "好きだ".
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2011-07-11T15:41:20.583
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2011-07-11T15:41:20.583
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It's hard to answer this specific question without getting into the more
general topic of the ～のだ construction, which, as jkerian mentioned, can mark
an explanation for a certain context, which may be either explicit or
implicit. Put succinctly, ～のだ provides supporting information. This
information is often a reason, but it may be a cause, basis, conclusion,
restatement, or confession. Because it is supporting, ～のだ tends to shift the
focus away from the information it marks. (This is in contrast to ～から, which
often draws more focus toward the information it follows.)

(Bill Nye voice) Consider the following:

> A: (1) You weren't in the office last week, were you? (2) Did you go
> somewhere?
>
> B: (3) Yeah, I traveled with my family to Nagoya.
>
> A: (1) 先週、会社にいなかったね。 (2) どこかに行った **のか** 。
>
> B: (3) うん、家族で名古屋に旅行してきた **んだ** 。

In sentence (2), A asks a question with ～のか, the question form of ～のだ. This
connects the question to sentence (1) and shows that the answer to the
question will provide a basis for the fact that B wasn't in the office last
week. (Sentence (2) could actually be left out entirely. In this case, B may
anticipate sentence (2) as an unspoken question and answer with the exact same
sentence (3) as above.) In sentence (3), B provides the supporting information
that A is looking for, and since it is supporting information, it is marked
with ～のだ (～んだ in speech).

From an English perspective, a sentence without ～のだ looks exactly like one
with ～のだ, because in English we tend to leave these inter-sentence connections
unspoken. This is part of why the ～のだ construction is difficult to grasp. You
could theoretically force an English rendition, but at the expense of
naturalness:

> A: (1) You weren't in the office last week, were you? (2) **[Is this]
> because** you went somewhere?
>
> B: (3) Yeah, **[I wasn't in the office] because** I traveled with my family
> to Nagoya.

Now from general to specific. In your question, it's difficult to say why the
speaker would use 好きなんだ over 好きだ, since we have no context. But ～のだ is often
used in confessing something the listener would have a hard time knowing.
(This also counts as supporting information, since its purpose is to fill in a
gap in the listener's understanding.) So 好きなんだ would be preferable to 好きだ in
that context:

> ずっと前に気づいたけど、今まで言えなくてごめん……実は、○○くんのことが好きなんだ… （胸キュン！）

好きなんだ is better here because the first half raises a question in the
listener's mind (What hasn't she been able to say until now?). Since the
second half is designed to anticipate and answer that question, it's marked
with ～んだ. 好きだ would technically convey the same information, but without の/ん
you lose that connection between thoughts and the sentence doesn't flow as
well in Japanese.
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2011-07-11T19:31:11.213
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2020-06-17T08:18:27.500
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I also would like to add my information gained from my teacher at college. He
says while the above explanation describes in great details in what context we
use 「んだ」 or 「のだ」, basically it cannot be translated as the expression of
cause, basis, conclusion, restatement, or confession. According to his
academic research on Japanese language 「んだ」 or 「のだ」only accidentally
explainable with these meanings, since 'cause, basis, restatemen and
confession' aren't expressed by them, but by the actual questions and answer
pairs. He says the real function of 「んだ」 or 「のだ」 regards the style of speech
rather than the content. The grammar in question is like the use of arms in
martial arts when we fall to the ground to blunt the power of the drop.
According to his findings, these sentence ending structures have the same
function, to blunt the edge of the question - for example - if we accidentally
conjugate the sentence ending verb into a short form.
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2016-08-01T08:54:19.633
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2016-08-01T08:54:19.633
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