こそ is a rather straightforward particle to understand, but its usage is a little tricky.
The first instances that students typically see こそ is in set phrases like こちらこそ (likewise), 今度こそ (surely next time), and 今年こそ (surely next year). However, even these set phrases can get messed up.
1a. 今年こそ危機に瀕する言語の重要性が分かるようになりました。 X
I've finally understood for the first time this year the importance of endangered languages.
If you strive like him, you’ll be able to go to Tokyo University.
こそ strongly emphasizes a person, thing, or situation in the sense of ほかでもなくこれなのだ. It can be seen after nouns (especially those concerning people), time phrases, Verb＋て, and the連用形 of verbs.
The entrepreneurial spirit is certainly the least risky road to take.
His demeanor is indeed courteous, but his true nature is mean.
The birds will certainly flap tomorrow.
Let’s get started now!
You'll certainly do better next time!
A friend in need is a friend indeed.
This year, indeed, let's try going to Japan.
I failed in trying to escape this damned uterus, but next time I will definitely come clear out of this hell and attain freedom!
Sentence Note: Someone like Stewie from Family Guy would say something like this.
A nation is born with the existence of a healthy citizen body.
There’s nothing to be angry about when you’re joyous.
Grammar Note: ～こそすれ・こそなれ・こそであれ is a retention of Classical Japanese grammar. In the past, the particle こそ was a bound particle, which made the final verb obligatorily in the已然形. When こそ is used after a verbal element or in a copular expression, this grammar still applies.
She does speak highly (of others), but she never criticizes.
So long as the woman is a fool, “good wife, wise mother” means nothing.
Do more harm than good.
Word Note: When you take out こそ in にこそなれ, you get the archaic copula にあり・なり.
Because こそ is so emphatic, sentences with it often end in ～だ, ～（よ）う, ～べきだ, etc. It is used a lot, but it is more common in the written language. Reasons for this include sentences like the last where traditional grammar holds on. In the spoken language, it tends to be used in statements by politicians and what not, and a few of the sentences above could definitely be used as slogans.
If you use が with こそ, it should be after it. を can go either before or after, and the particles に, へ, で, と, から all come before.
We want to recommend Mr. Yamada to the governor.
There is indeed delicious food in Japan.
Now I can fly, but at that time, I couldn't whatsoever.
Unlike using が, which sounds like the speaker is picking one thing as the focus, こそ emphasizes this “focus” as the sole thing fit for the situation. It is often used in sentences where one quality is highlighted in the first clause with こそ, but then it gets negated. When this gets flipped around, the second clause with こそ shows what’s actually the case.
18. 「オバマ大統領の支持率が低いですね。」「いえ、違いますよ。支持率こそ伸び悩んでいますが、何かやってく れそうですよ。」
“President Obama’s approval rating is low, isn't it?” No, you’re wrong. Though his approval rating may very well be lagging behind, he seems like he’s going to do something for us”.
They say money is the root of evil, but it’s the lust for money that is the root of evil.
Using ～こそが instead of ～こそ can’t be easily explained. If a noun phrase being modified by こそ is the subject of the sentence, grammatically speaking, there is nothing wrong with using ～こそが. If the particle is dropped, it may be because other things in your sentence are just off for the tone to work.
Spaying and neutering will definitely prevent the rapid increase of the dog population. As for cats, poison laced milk should do it.
～からこそ, not surprisingly, is going to like “indeed, because…”. Though this may seem like a choppy translation in isolation, something is being pointed out as the reason with から, and when you addこそ, the reason becomes all the more emphasized.
And therefore, in order to create a strong Japan, let us fix the constitution!
One can only assume that the Japanese speaking couple was in America to cause such an emotional parting, but putting that aside, one should clearly see how ～からこそ works.
AからこそB is very similar to AばこそB, and they are often interchangeable. However, there are enough differences to make them not interchangeable. Both are used when the speaker compares the situation with past experience or knowledge and can’t think of anything but reason A for result B.
It's precisely because I’m thinking about you that I say this.
Even if you've committed a crime in the past, it is precisely because you are giving your all to the better of people that you will be rewarded by God.
It is exactly because I love you that we must separate.
It is precisely because of your short temper that you mustn't get into an argument.
The reason why I am able to live fulfilled each day is the fact that my mind and body are healthy.
It was precisely because Michiko had that conception that she was for now able to throw herself to the base in the mountains and end her cohabitation lifestyle.
From 光の雨 by 立松和平.
Word Note: 身を投ずる ＝ 身を投じる.
～ばこそ has become more literary, and it is usually not used by the younger generations. Inverting the sentences can cause grammatical problems. When ～からこそだ ends the sentence, it is often the case that こそ just gets dropped. Sometimes, keeping it can make the sentence sound unnatural, especially in the spoken language. Here, the particle needs formality to make it work.
Examples with the format A｛から・ば｝こそB can be reworded to BのはA｛から・ば｝こそだ if the context is resultative, but in contradictory contexts, you cannot. However, when you can reword to BのはAからこそだ, A it is because A is expressing a positive cause/reason. Negative situations don't go.
～ばこそ may be used in showing a general condition. This is simply based on the fact that you are using the conditional particle ば in the pattern. As stated above, the context does not involve contradictory clauses. ば functions as a general condition, and the situation is in regards to knowledge, morals, or logic of some sort. The pattern becomes unnatural when the situation is already a defined condition of the past.
29. 政府のことを｛思ったからこそ 〇・思えばからこそX｝、腹を立てたんじゃないか。
It's surely because you thought about the government that you got mad, no?
Of course, when you think about the government, you often get mad.
Because I certainly have a true friend to encourage me on, I can live on without setbacks.
When A＋｛から・ば｝こそB are expressing the speaker’s judgment, it can only be used in declarative sentences or questions as what should be. When you fall out of the laws of nature and dispensation, even if you use a general condition, these phrases become unnatural.
Because I truly believe in you, I am asking you this.
Because I truly think about Okada-san, should I tell him/her the truth?
Since there is also gravity on Mars, objects will fall to the surface.
34. 表題作の「光と影」も、まさに医家でなければもてない眼差しと、作家でなければ見抜くことができない眼差し があったればこそと、私は確信している。
Along with the title work "Light and Shadow", I'm confident that he had the eye that only a doctor could have and the eye with which he could see through things being a novelist.
From 光と影 by 渡辺淳一 in the 解説 by 小松伸六.
Grammar Note: あったれば ＝ あれば.
AてこそB is used when the speaker is evaluating or persuading the listener based on experience/social wisdom/morals/ethics and a positive B coming about. Because there is a condition A, a positive B naturally/necessarily comes about. B must be some noun (of condition), adjective, potential or passive phrase, all of which are non-volitional. B can’t be a phrase that shows the speaker’s/your volition.
As you cheerfully welcome strangers, you will be warmly welcomed by anyone.
36. 身を捨ててこそ浮ぶ瀬もあれ。(Set Phrase)
Literally: Just as throw your own self away, there are also rapids for your body to float in.
Risk all and gain all.
37. この山中に暮らしをしてこそ、完璧に平等な理想の社会を築くことができるのではないか。人間らしい叫び声 を上げることができてこそ、革命家たり得るはずなのだ。
Isn't it not possible to build a perfectly impartial society by living within the mountains? It should be possible to become a revolutionist by simply giving a human shout.
From 光の雨 by 立松和平.
One can grasp things as phenomena so long as there is a cause and condition, but conditions change moment by moment.
From 光の雨 by 立松和平.
Grammar Note: たり得る is a combination of たり (a classical copula) and ～得る (showing potential). It is equivalent to であることができる。
All instances of ～てこそ may be replaced with ～てはじめて, but this doesn't mean that ～てはじめて is always interchangeable with ～てこそ. Just like ～ばこそ, ～てこそ cannot be used with some past or individual event with a defined condition.
Since meeting you, I feel like I've understood the meaning of true love for the first time.
AてこそB is interchangeable with A｛から・ば｝こそB whenever the situation is a defined, resultative condition, the likes of which are seen with から・ので. However, if the situation is hypothetical, you cannot paraphrase ～てこそ out with just から. It should be ～てからこそ・ばこそ・ば. If it is a situation that could be defined or hypothetical, paraphrasing ～てこそ out with ～てからこそ, ～ばこそ, ～ば, or ～から is fine.
It is precisely because of you giving your all (for something) that you will be rewarded by God.
41b. 人々がこの地球を｛守ってからこそ・守ればこそ｝、人類はサバイバルが可能だ。〇人々がこの地球を守るか ら、人類はサバイバルが可能だ。X/△
So long as people protect this earth, humanity has the potential to survive.
You'll succeed if I try all my might.
Of course, by now you should realize that there are minor nuance changes in switching between the options, but at least you know what a situation looks like in which all are fine.