こそ details
N and no other — singles its noun out against everything else in play
努力こそが人を変えるのだと思います。
doryoku koso ga hito o kaeru no da to omoimasu.
It's effort, more than anything, that changes a person. neutral
今度こそ絶対に合格してみせる。
kondo koso zettai ni goukaku shite miseru.
This time for sure, I'm going to pass. casual
- alternant
Examples
N and no other is what counts — the noun is picked out of a field of candidates and the predicate is staked on it
努力こそが人を変えるのだと思います。
doryoku koso ga hito o kaeru no da to omoimasu.
It's effort, more than anything, that changes a person. neutral
困っているときこそ、周りに声をかけたほうがいいよ。
komatte iru toki koso, mawari ni koe o kaketa hou ga ii yo.
It's exactly when you're stuck that you should say something to the people around you. casual
こちらこそ、今後ともよろしくお願い申し上げます。
kochira koso, kongo tomo yoroshiku onegai moushiagemasu.
The pleasure is mine — I look forward to working with you. formal
今こそ、みんなで力を合わせるときです。
ima koso, minna de chikara o awaseru toki desu.
Now, if ever, is the time for us all to pull together. neutral
this time for sure — on 今度, 次 or 来年, こそ puts the speaker's resolve on the coming occasion after the earlier ones failed
今度こそ絶対に合格してみせる。
kondo koso zettai ni goukaku shite miseru.
This time for sure, I'm going to pass. casual
次こそは負けません。
tsugi koso wa makemasen.
Next time I won't lose. neutral
Formation
{N}こそattaches to noun格助詞＋こそ（君にこそ・ここでこそ）attaches to noun phrase (schematic)
Variant notes
- こそが — が comes back after こそ when the singled-out noun is the subject and the sentence wants that marked — 努力こそが人を変える; は returns the same way (次こそは), and bare こそ says the same thing and is the commoner shape in speech.
Notes
too (宿題こそ先に終わらせた) but survives in formal
writing. に and で keep their place and こそ follows them — 君にこそ,
never ×君こそに.
Related patterns
Similar
からこそ is this same こそ clamped onto a reason clause, so it argues about WHY (難しいからこそやりたい); bare Nこそ argues about WHICH — it picks the noun out of a field of candidates and leaves the reasoning unstated, which is why it needs no のだ tail where からこそ almost always takes one.
てこそ names an action as the one precondition that makes the result possible, and the result has to follow in the same sentence; Nこそ names an entity and can close the sentence on the spot (こちらこそ), demanding no consequence clause at all.
Lookalikes
Both swallow が／は after a bare noun and both read emphatic, but they emphasise in opposite directions — さえ picks the LEAST likely member to argue the rest follows (子供さえ分かる), こそ picks the one member it wants and rules the rest out (今こそ動くときだ); read one for the other and the sentence's argument inverts.
も and こそ sit in the same position after a bare noun and both displace が／は, so they look interchangeable — but も adds its noun to what is already granted (私も行く "me too") while こそ pushes the others out (私こそ行く "I'm the one who should go").