こそ is a 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” or “for sure,” as in:
Emphasis (Noun + こそ)
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言わざるを得ない。
That—I have to say—is wrong.
彼こそが天才です。
He is the one that is a genius.
Important Rule: This expression is not used with negative verbs. Because こそ strongly asserts the positive existence or truth of something, it sounds unnatural to use it to emphasize a negative action.
納豆こそ食べられない。(✘)
[I cannot eat natto]
Especially Because (からこそ)
The particle is easier to understand when used with the reason particle から. Here, it is used to place heavy emphasis on the reason for the second clause, translating to “precisely because” or “especially because.”
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 precisely because I had someone 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 especially 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 discuss it with friends.