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