Grammar Info
N3 Lesson 3: 2/23

こそ
for sure, for certain, even more so, underlined/bolded text

こそ replaces particles を and が.

Structure
Noun + こそ
Details
Register Standard
使用域 一般
Information
こそ is a 副助詞(ふくじょし) (adverbial particle) in Japanese that primarily replaces either or が. Its main role is to highlight the preceding noun (or noun phrase) as being the the most important piece of information in relation to the rest of the sentence. In English, this is an effect that is primarily achieved through things like bold text, underlining, or emphasizing a particular word in speech. If we were to make a direct comparison to an English statement, then things like ‘precisely because it’s (A)’, or ‘rather it is due to (A)’, are quite similar to こそ’s function.
こそ can be used directly after any noun that would usually take the , or が particles, so long as that particular piece of information needs to be emphasized.
  • この曲(きょく)こそ俺(おれ)ずーっと探(さが)していた曲(きょく)!やっとタイトル分(わ)かった。
    This song is exactly the song I have been searching for! I finally figured out the title.
  • 今度(こんど)こそ勝(か)つぞ!
    I will win for sure next time!
Caution
The reason that こそ is able to replace が, but not は, is due to the fact that が primarily highlights specific pieces of information, while は nominates general topics. This means that they have completely opposite functions. To make things easier, the three particles can be thought of in the following way:
は- Weakly highlights specific information.
が- Strongly highlights specific information.
こそ- Very strongly highlights specific information.
Fun Fact
こそ is often used after names or pronouns, as a way to reply to thanks/apologies. In these situations, it expresses that the speaker feels equally (or more) obliged to say what was just said to them by someone else.
  • いえいえ、私(わたし)こそありがとうございます
    No no no no, I am the one that should be thankful.
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あなたこそできる。
You can do it!
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いえいえ、謝(あやま)らないでください!私(わたし)こそ悪(わる)いことをしてしまいました。
No, no! Please do not apologize. It was (more) me (than you) that was in the wrong.
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一人(ひとり)でも手助(てだす)けをすることができたら、それこそ最高(さいこう)の気持(きも)ちになる。
Even if we can only help one person, that (even more so) is the best feeling.
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Books
Tobira
Page 284
[AIAIJ] An Integrated Approach to Intermediate Japanese
Page 171
Quartet II
Page 11, Point 1
A Handbook of Japanese Grammar Patterns
Page 118
A Dictionary of Intermediate Japanese Grammar
Page 132