Precisely, this very one: 〜こそ

こそ puts strong emphasis on a word: “it's precisely…, this very… (and nothing else)”. 今年こそ合格する: “this year's the year, I'm going to pass”. こちらこそ: “no, it's me (who should thank you)”. からこそ underlines the cause: “precisely because”.

• 今年こそ、試験に合格したい。ことしこそ、しけんにごうかくしたい。This year's the year: I want to pass the exam.
• 「よろしくお願いします。」「こちらこそ、よろしくお願いします。」「よろしくおねがいします。」「こちらこそ、よろしくおねがいします。」“Nice to meet you.” “Likewise, the pleasure's all mine.”
• 好きだからこそ、厳しいことを言うんだ。すきだからこそ、きびしいことをいうんだ。It's precisely because I care about you that I say harsh things.

こそ is a very strong emphatic particle: it singles out one element and says “this one, and no other”. It replaces は or が and adds emotion: determination, conviction, sincerity. In English you'd stress the word with your voice, or add “precisely”, “this very”, “this is the one”.

You meet it in set phrases (こちらこそ, “no, it's me who…”) and in resolutions (今年こそ, 今度こそ, “this time's the time”).

• 今度こそ、うまくいくと思う。こんどこそ、うまくいくとおもう。This time, I think it'll work.

こそ replaces は, が and を. With other particles (に, で, から…), you keep the particle in front: ここでこそ, あなたにこそ. With a cause, からこそ (“precisely because”); with a verb in て, 〜てこそ (“only by… can you”): 自分でやってこそ、身につく.

• 自分でやってこそ、本当に身につく。じぶんでやってこそ、ほんとうにみにつく。It's only by doing things yourself that you really master them.
• 京都でこそ見られる景色がある。きょうとでこそみられるけしきがある。There are views you can only see in Kyoto.

は presents a topic, が marks the subject. こそ does both with maximum emphasis, often to contrast with what came before (“not last time, THIS time”) or with what the other person thinks (“no, it's ME”).

からこそ is stronger than から: it answers an expected opposite idea. 忙しいからこそ休む, “it's precisely because I'm busy that I rest” (when you'd think the opposite).

• 忙しいからこそ、休む時間が大切だ。いそがしいからこそ、やすむじかんがたいせつだ。It's precisely because you're busy that time to rest matters.
• あなたにこそ、この仕事をお願いしたい。あなたにこそ、このしごとをおねがいしたい。You're precisely the person I'd like to entrust this job to.