〜こそ

こそ singles out one element and emphasizes it as the true or most important one, often implying a contrast with something else. With からこそ it stresses the real reason ('it is precisely because...'), and with てこそ it means a result is only achieved when that condition is met. It is an emphatic particle, not a conjugation, so it replaces は/が after a noun; it is common in both speech (こちらこそ) and formal writing.

• こちらこそ、よろしくお願いします。

  こちらこそ、よろしくおねがいします。

  The pleasure is mine — I look forward to working with you.

• 今年こそ日本語能力試験に合格したい。

  ことしこそにほんごのうりょくしけんにごうかくしたい。

  This year for sure, I want to pass the JLPT.

• 努力したからこそ、今の成功があるのだ。

  どりょくしたからこそ、いまのせいこうがあるのだ。

  It is precisely because he worked hard that he has this success today.