Gray-san-ga yoku junbi-shita-kara-koso, umaku itta-n-da-yo. (It went well because you prepared properly, Ms. Gray.)

Situation 1: Section chief Mr. Okubo praises Ms. Gray for delivering a successful presentation.

大久保： 販売促進のプレゼンは大成功だったね。

グレイ： 今回は製品に関心を持つお客さんが大勢いらしたからじゃないでしょうか。

大久保： いや、グレイさんがよく準備したからこそ、うまくいったんだよ。

Ōkubo: Hanbai-sokushin-no purezen-wa dai-sēkō-datta-ne.

Gray: Konkai-wa seihin-ni kanshin-o motsu o-kyaku-san-ga ōzei irashita-kara-ja-nai-deshō-ka.

Ōkubo: Iya, Gray-san-ga yoku junbi-shita-kara-koso, umaku itta-n-da-yo.

Okubo; The sales promotion presentation was a great success!

Gray: I suppose that's because a lot of customers who were interested in this product came here today.

Okubo: No, it went so well because you prepared properly, Ms. Gray.

Today, we will introduce XからこそY and XてこそY, two patterns that are used with the particle こそ. In the pattern Xこそ, こそ is the particle that emphasizes X and makes it stand out from others. In the XからこそY pattern, X(=clause in plain form) is the reason that the speaker thinks is the most important, and Y is the result of X. Therefore, XからこそY is not used to show an objective relationship. This result, Y, is usually positive, and the sentence end of Y is often んです/んだ, as in Mr. Okubo's remark. Example: 皆(みな)さんのご支援(しえん)があったからこそ、がんばることができたんです (Because of your support, I was able to give my best). X(=reason)からこそY(=result) often expresses a result that goes against common sense. Example: 映画(えいが)が好(す)きだからこそ、映画を仕事(しごと)にはしたくなかった (Because I like movies, I didn't want to get a movie-related job).

Situation 2: Takako, a junior high school student, complains to her mother about her math homework.

たか子： 今日の宿題、めんどくさいの。こんなの、パソコンを 使えば簡単なのに。

母： 自分の頭で考えてこそ計算の仕方がわかるのよ。

Takako: Kyō-no shukudai, mendokusai-no. Konna-no, pasokon-o tsukaeba kantanna-no-ni.

Haha: Jibun-no atama-de kangaete-koso keisan-no shikata-ga wakaru-no-yo.

Takako: Today's assignment is such a hassle. It would be easy to do on a PC, though.

Mother: You'll only understand how to calculate if you think it through by yourself.

X(=verb in te-form)こそY means that only after/when X occurs can Y be realized, so X is a sole condition of Y. The speaker shows that only condition X will realize the positive result Y. Here, Y is usually the non-past potential form or わかる. X(=verb in te-form)こそY can be interchangeable with X(=verb in te-form)はじめてY. Example: 外国(がいこく)に住(す)んでこそ、自分(じぶん)の国(くに)の文化(ぶんか)が理解(りかい)できると思(おも)う (I think that you can only understand the culture of your country by living abroad).

Bonus Dialogue: Mr. Sere talks to Mr. Mita about Mita's girlfriend. (ドタキャンする is a slang term meaning to cancel something at the last minute.)

セレ： この前(まえ)も彼女(かのじょ)との約束(やくそく)をドタキャンされたんだって？

三田： うん、仕事(しごと)が忙(いそが)しかったって言(い)ってたよ。

セレ： そうかなあ。あの日(ひ)はパーティーに行(い)ってた らしいよ。

三田： 彼女、忘(わす)れっぽいんだよ。

セレ： ふうん...。友(とも)だちだからこそはっきり言(い)う けど、あの人(ひと)はやめたほうがいいんじゃない？

三田： 彼女は欠点(けってん)があるからこそ、かわいいと思 (おも)うんだ。ぼくがいるからこそ、彼女はあんなふうに楽(たの)しそうにしていられるんだよ。[たちさる]

セレ： [ひとりごと]やれやれ。失恋(しつれん)をしてこそわかることがあるからなあ。

Sere: She canceled her appointment with you at the last minute again the other day, right?

Mita: Yeah. She said she was busy with work.

Sere: I wonder ... I heard she went to a party that day.

Mita: She's forgetful.

Sere: Hmm ... I'll say this frankly and precisely because I'm your friend: You should split up with her.

Mita: I love her all the more for her faults. She can be happy like that only because I'm her boyfriend. [He leaves]

Sere: [To himself] Oh no. He won't understand until he has his heart broken.