〜こそすれ

This pattern is a remnant of classical Japanese grammar, specifically the rule of 係り結び（かかりむすび）. Under this rule, the emphatic particle こそ forces the verb at the end of the clause into its realis form（已然形、いぜんけい）. This explains exactly why する becomes すれ, ある becomes あれ, and なる becomes なれ. It is highly formal（硬い）and is typically found in literature, editorials, or formal speeches. It translates to 'I may do A, but I certainly don't do B' or 'A, but definitely not B.'

1. Structure:

• Verbs: Use the ます-stem + こそすれ + [Negative Clause] (e.g., 増え（ふえ）こそすれ、減る（へる）ことはない - It may increase, but it will never decrease).
• Suru-Nouns: Use the action/emotion noun + こそすれ + [Negative Clause] (e.g., 感謝（かんしゃ）こそすれ、恨む（うらむ）ことはない - I may feel grateful, but I would never hold a grudge). Note that you can also apply the standard verb rule to suru-verbs, resulting in Noun + しこそすれ (e.g., 感謝しこそすれ).

2. Nuance & Usage Rules:
The speaker uses this to correct a potential misunderstanding or to state a firm conviction. By saying 'A happens,' they are setting the stage to say 'but B (the opposite) absolutely does not.' The words used in the two clauses are almost always direct antonyms or highly contrasting pairs (e.g., 増える vs. 減る, 感謝 vs. 恨む). While the subject of both the first and second clauses is usually the same, they can differ (e.g., 雨が降りこそすれ、雪が降ることはない). Additionally, the pattern usually pairs with strong negative endings in the second clause, such as 〜ことはない, 〜はずがない, or 〜絶対にない.

3. Action vs. State:
It is important to distinguish this from 〜こそあれ. The distinction is based on the underlying verb: use 〜こそすれ when the meaning is 'to do X' (Xはするが), and 〜こそあれ when the meaning is 'X exists' (Xはあるが). Action nouns can take 〜こそあれ if the focus is on existence (e.g., 苦労（くろう）こそあれ、...).

Furthermore, unlike 〜こそすれ, 〜こそあれ does not require a negative clause in the second half and is frequently followed by a positive statement (e.g., 苦労こそあれ、子育ては楽しいものです - There may be hardships, but raising children is fun). When using 〜こそあれ with adjectives, い-adjectives take the く-form (e.g., 高くこそあれ - It may be expensive, but...) and な-adjectives take で (e.g., 便利でこそあれ - It may be convenient, but...). Regular nouns can also take でこそあれ to express state or identity (e.g., 冗談（じょうだん）でこそあれ - It may be a joke, but...). For changes in state (becoming something), 〜こそなれ is used, which attaches to Noun + に or the に form of な-adjectives (e.g., プラスにこそなれ、マイナスにはならない - It may become a plus, but it will never become a minus).