кто бы ни / что бы ни

The construction кто бы ни / что бы ни is a powerful tool for expressing concessive conditions. While this explanation focuses on кто and что, this universal pattern applies to the entire family of interrogative words (e.g., где бы ни - wherever, когда бы ни - whenever, какой бы ни - whichever).

1. Structure:

• Pronoun: Use кто (who) or что (what). These must decline to match their role in the clause (e.g., кого бы ни - whomever, чему бы ни - to whatever).
• Prepositions: If a preposition is needed, it must be placed directly before the interrogative pronoun, not before бы or ни (e.g., С кем бы ни... - With whomever..., О чём бы ни... - About whatever...).
• Agreement Rule: The pronoun кто typically requires the verb to be in the masculine singular form (e.g., Кто бы ни пришёл... - Whoever might come...). However, when кто or что acts as the predicate, the verb agrees with the actual subject in both number and gender, and the pronoun is highly naturally used in the instrumental case (e.g., Кем бы ни была эта женщина... - Whoever this woman was..., Чем бы ни была эта вещь... - Whatever this thing was...), although the nominative is also possible (e.g., Кто бы ни была эта женщина... - Whoever this woman was...).
• Particles & Word Order: For this specific conditional construction, the combination of бы and ни is mandatory. Note that ни here acts as an intensifier, not a simple negation. While бы most commonly follows the pronoun, it is a mobile particle and can also attach to the verb (e.g., Что он ни говорил бы... - Whatever he might say...). Importantly, ни is frequently separated from the pronoun and бы by a subject or object (e.g., Что бы он ни сказал... - Whatever he might have said...). Note that the concessive meaning can also be achieved without бы using other tenses or moods, such as the imperative (e.g., Что ни скажи, всё не так - Whatever you say, everything is wrong). Furthermore, dropping бы and using the standard indicative past or future changes the meaning from a hypothetical to a factual, repeated action (e.g., Что он ни говорил, ему никто не верил - Whatever he said, no one believed him).
• Verb: The verb is typically in the past tense form because of the particle бы, even if the meaning is future or hypothetical.

2. Meaning:
It translates to 'no matter who/what' or '-ever'. It implies that the specific identity of the person or object does not change the truth of the main clause. Highly common fixed idioms using this pattern include во что бы то ни стало (at any cost / no matter what), кто бы то ни был (whoever it may be), and что бы то ни было (whatever it may be), where the particle то is frequently inserted into the middle of the pattern.

3. Spelling and Punctuation:

• Ни vs. Не: It is crucial to distinguish between the concessive кто бы ни (whoever) and кто бы не. Use ни to generalize a statement (e.g., Кто бы ни звонил, я занят - Whoever calls, I am busy). Use не for actual negation, such as in relative subordinate clauses containing double negation (e.g., Нет никого, кто бы не знал этого - There is no one who wouldn't know this), or in independent rhetorical questions (e.g., Кто бы это не понял? - Who wouldn't understand this?).
• Что бы ни vs. Чтобы не: In this concessive construction, что бы ни (whatever) must be written as three separate words. This is a critical distinction from the purpose conjunction with negation чтобы не (so as not to / in order not to), where чтобы is written as one word.
• Punctuation: These constructions form subordinate clauses and must always be separated from the main clause by a comma.