こそ
1
particle
it is ... that ...;precisely;in particular;definitely;for sure;only (when, after, because, etc.)
emphasizes preceding word or phrase
辛抱こそ大事なんだ。
It's patience that counts.
2
particle
although;while;it is the case that ... but
as こそあれ, こそすれ, こそするが, etc.
彼は年こそ若いが十分その仕事をやっていける。
Young as he is, he is equal to the task.
3
particle
it is precisely because ... that ...;only because ...
after conditional -ba form of verb
君のためを思えばこそこう言ってるんだ。
It is precisely because I have your best interests at heart that I am saying this.
4
particle
not at all;not in the slightest;absolutely not;never
after -nai stem of verb + ば