の details
write + it's that…
kaku no
it's that…; is it that…? — the plain-style explanatory ending
ちょっと頭が痛いの。
chotto atama ga itai no.
It's just that I have a bit of a headache. casual
どこ行くの？
doko iku no?
Where are you off to? casual
- dictionary form
- past
- negative
- negative + past
- na-adjective
- noun
Examples
explaining — the sentence accounts for something the listener can already see
ちょっと頭が痛いの。
chotto atama ga itai no.
It's just that I have a bit of a headache. casual
明日から出張なの。
ashita kara shutchou na no.
I'm away on a business trip from tomorrow, you see. casual
asking after it — the same tail with rising intonation
どこ行くの？
doko iku no?
Where are you off to? casual
もう食べたの？
mou tabeta no?
You've eaten already? casual
Formation
{P}のattaches to plain form{NA-stem}なのattaches to na-adjective{N}なのattaches to noun
Notes
and feminine, where men reach for 〜んだ and the question is gender-neutral.
Nouns and na-adjectives link with な in the non-past (学生なの); the
past keeps だった (学生だったの).
Related patterns
Similar
The same explanatory frame at two registers — の is the plain-style ending and stops there (どうしたの), んです carries it into です／ます speech (どうしたんですか); 行きますの survives only as dated, refined-feminine speech, so the polite version is んです and never a politer の.
Both close a spoken question, but か asks flatly for a fact and works in any register, while の asks after the situation behind what the speaker can see (泣いてるの？) and stays plain-style — か on a plain form is blunt where の is soft.
Lookalikes
The same kana doing opposite jobs — sentence-final の ends the clause (泳ぐの？), while the nominalizing の heads it into a following predicate (泳ぐのが好き); if a predicate follows the の, it is not this pattern.
Same kana, different job — between two nouns の links them (私の本), at the end of a finished sentence it is the explanatory particle asking or accounting for something (どうしたの？), and no noun follows it there at all.
What follows decides — the nominalizer hands its clause to a particle and a predicate (泳ぐのが好きだ), while sentence-final の closes the utterance and nothing but a final particle or intonation follows it (どうしたの・知らないのよ).