it is ... that ...; precisely; definitely
Emphasizes the preceding word or phrase, often contrastively. Common in patterns like ～こそ～が, ～からこそ, and set phrases such as こちらこそ.
This year, I definitely want to master Japanese.
precisely because; only because
Follows the conditional -ba form of a verb, emphasizing that the reason given is the very cause. Often used in explanations or justifications.
It is precisely because you make an effort that success comes.
not at all; absolutely not; never
Formal or literary. Follows the -nai stem of a verb + ば, creating a strong negative expression. Often used in emphatic denial.
You are exactly the right person for this job.
It is I who should say that; pleased to meet you.
Although there are hardships, what you gain is also great.
His determination will not waver in the slightest.
It's precisely because he's a child that some things are forgiven.
Precisely because I went through hardship, the joy of success is greater.
I'm not exactly opposed, but I'm not in favor either.
さえ marks a minimal or extreme example, meaning 'even', while こそ singles out an element as precisely the key point.
だけ means 'only' and restricts scope, whereas こそ emphasizes an element as the very thing being identified without limiting it.
The origin is uncertain, but こそ is an Old Japanese emphatic particle, likely from a combination of the proximal demonstrative こ (this) and an emphatic element そ.