こそ
koso

Particle

1. it is ... that ...; precisely; in particular; definitely; for sure; only (when, after, because, etc.)

辛抱 しんぼうこそ大事 だいじなんだ。
shimbōkoso daijina nda.
It's patience that counts.
こだわりはお きゃく つたわってこそ意味 いみをなす。
kodawariwa okyakuni tsutawattekoso imio nasu.
Care over the particulars only has meaning once it gets across to customers.
Particle

2. although; while; it is the case that ... but

かれ としこそ わかいが十分 じゅうぶんその仕事 しごとをやっていける。
karewa toshikoso wakaiga juppun sono shigotō yatte ikeru.
Young as he is, he is equal to the task.
Particle

3. it is precisely because ... that ...; only because ...

きみのためを おもえばこそこう ってるんだ。
kimino tameo omoebakoso kō itteru nda.
It is precisely because I have your best interests at heart that I am saying this.
Particle

4. not at all; not in the slightest; absolutely not; never

Formal Register