A particle used to strongly emphasize the preceding word or phrase (equivalent to “precisely,” “indeed,” or “it is ... that ...”).
これこそ私が長年探していた本だ。
This is precisely the book I've been looking for for years.
彼こそ先生にふさわしい人物だ。
He is the person who is truly suitable to be the teacher.
今こそ行動を起こすべきだ。
Now is precisely the time to take action.
Used in patterns like “〜こそあるが/〜こそすれ” to admit the preceding point while contrasting it with something else (meaning “although X is true, ...”).
彼は親切なこそあるが、頼りになるかは別だ。
He is kind, to be sure, but whether he can be relied on is another matter.
値段が安いこそ安いが、品質が心配だ。
True, it's cheap, but I'm worried about the quality.
彼は才能があるこそあるが、努力を続けないと伸びない。
He does have talent, but if he doesn't keep working hard he won't improve.
Follows the conditional -ba form (〜ればこそ) to mean “precisely because ... (therefore ...)”, emphasizing the reason or cause.
苦労すればこそ、人は強くなる。
It is precisely because one goes through hardship that people grow stronger.
助け合えばこそ、コミュニティは続くのだ。
It is precisely because we help each other that the community endures.
議論すればこそ良い結論が出る。
Only by debating (precisely because we debate) can a good conclusion be reached.
A formal or literary usage following the negative stem + ば (classical style), used to strongly express “not at all” or “absolutely not”; now rare and literary.
知らざればこそ、軽々に言うべきではない。
Precisely because one does not know, one should not speak lightly. (literary)
疑えぬばこそ、我々は慎重に振る舞わねばならない。
Because there is no doubt, we must act with caution. (literary/formal)