〜こそ
precisely, this one (for sure)
Noun + こそ · 〜からこそ · こちらこそ
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こそ puts strong emphasis on one thing: "this, and no other". 今年こそ、試験に合格したい, "this year, for sure, I want to pass the exam". It suggests previous years didn't work out.
The most common everyday use is こちらこそ, "no, thank you" or "the pleasure is mine", in reply to thanks or an apology.
からこそ stresses a reason: 失敗したからこそ、学べたことがある, "there are things I could learn precisely because I failed".
It's only for positive emphasis: you pick something out as the right one, the real one or the important one. こそ replaces は, が and を, and other particles go in front of it: 忙しい時にこそ.
Examples
今年こそ、日本語能力試験に合格したい。
This year, for sure, I want to pass the JLPT.
「ありがとう。」「こちらこそ。」
"Thank you." "No, thank you."
これこそ私が探していた本だ。
This is exactly the book I was looking for.
忙しい時こそ、休みが必要だ。
It's precisely when you're busy that you need a rest.
失敗したからこそ、学べたことがある。
There are things I could learn precisely because I failed.