JLPT N3. Used occasionally in conversation and writing.
こそ is particle used to emphasize and differentiate. It is appended to nouns and pronouns, very often the pronouns これ and それ and can take the place of the topic or subject particle. The closest direct translation is “that’s just the very thing” as in:
これこそずっと探していたものだよ。
This is just the very thing I’ve been looking for.
それこそ間違っているといわざるを得ない。
That—I have to say—is wrong.
彼こそが天才です。
He is the one that is a genius.
The expression is not used with the negative.
納豆こそ食べられない。(x)
[I cannot eat natto]
The particle is easier to understand when used with the からwhere is used to place emphasis on the reason for the second clause.
その辛い時期に頼れる人がいたからこそ、私は今こちらに立って話しています。
It was only because I had some I could turn to during that tough period that I’m standing here now speaking to you.
夢があるからこそ遅くまで働いていても構わないのです。
It’s because I have a dream that I don’t mind working late.
だからこそ友人と相談すべきでしょう。
That’s all the more reason why you should to discuss with friends.