какой бы ни

The construction какой бы ни is an advanced way to express concession. It follows a strict syntactic structure:

• Agreement: When modifying a noun, the word какой must agree with it in gender, number, and case. The entire noun phrase takes whatever case is dictated by the verb or preposition in that clause (e.g., Accusative: Какую бы книгу он ни читал... - 'Whatever book he reads...'). However, какой can also function as a standalone pronoun meaning 'whichever (one)', maintaining the same agreement rules based on its referent and syntactic role (e.g., Здесь много книг. Какую бы ты ни выбрал, она тебе понравится. - 'There are many books here. Whichever one you choose, you will like it.'). Alternatively, it takes either the nominative or the instrumental case when functioning as a predicate with 'быть' (e.g., Какой бы он ни был / Каким бы он ни был - 'Whatever he may be'); both cases are standard and frequently used in modern Russian. In formal Russian, the short-form pronoun каков is frequently used instead of какой in this predicative role (e.g., Какова бы ни была причина... - 'Whatever the reason may be...').
• Prepositions: If a preposition governs какой and its noun, it must be placed immediately before какой (e.g., В каком бы... - 'In whatever...'). If the preposition belongs to a different noun phrase later in the clause, it stays where it belongs.
• The Subjunctive and Tense: The particle бы triggers the subjunctive mood, which typically uses the past tense of the verb regardless of the actual time frame (or the infinitive in impersonal constructions). Meanwhile, the main clause uses the actual tense (past, present, or future) to establish when the event happens. The main clause frequently uses a correlative emphasizing word like всё равно, всё же, or всё-таки to complete the thought.
• The Particle 'ни': Unlike the negative particle 'не', ни here generalizes the statement to include all possibilities ('no matter what kind...'). It must be placed immediately before the verb. Because какой бы ни introduces a full subordinate concessive clause, a predicate is strictly required; omitting the verb (e.g., leaving ни dangling before a noun) is ungrammatical. However, the phrase какой бы то ни было has lexicalized into an adjectival phrase meaning 'any whatsoever'. Although it is a set phrase, the pronoun какой inflects to agree with the modified noun in gender, number, and case. The verb form было is frozen in the neuter singular past tense and is invariable in all cases (e.g., какая бы то ни было причина, без какого бы то ни было сомнения). Because it acts as a modifier rather than introducing a subordinate clause, it doesn't require a new predicate.
• 'ни' vs. 'не': It is crucial to distinguish the concessive какой бы ни ('no matter what kind') from a relative clause with true negation какой бы не ('which would not'). While both particles are often unstressed clitics and homophones (pronounced [nʲɪ]) in rapid speech due to vowel reduction, не can carry secondary logical stress to emphasize negation, in which case it is pronounced clearly as [nʲe]. For example, compare Какой бы фильм он ни смотрел, ему всё скучно (No matter what movie he watches, he is bored) with Нет такого фильма, какой бы он не смотрел (There is no movie that he hasn't watched). Because they sound identical in unstressed positions, writing какой бы не instead of какой бы ни in a concessive clause is a very common spelling mistake, even among native speakers.
• 'какой бы ни' vs. 'какой ни': While какой бы ни uses the past tense (subjunctive) for hypothetical or general situations, you can also drop бы to form какой ни. This variant uses the indicative mood (present or future) or the imperative (e.g., Какую книгу ни возьми... - 'Whichever book you take...').
• Word Order: The какой бы [noun] phrase typically fronts (starts) the subordinate clause, acting as a relative pronoun phrase. The particle бы typically immediately follows какой, though it is a highly mobile clitic that can also grammatically follow the modified noun, the subject (e.g., Какую книгу он бы ни читал... - 'Whatever book he reads...'), or the verb. Furthermore, the modified noun's placement is highly flexible: it can appear immediately after какой, split the construction, or even follow the verb (e.g., Какую бы он ни читал книгу... - 'Whatever book he reads...'). Additionally, subject-verb inversion is highly natural and common in these concessive clauses (e.g., Какую бы книгу ни выбрал студент...). The typical order is [Preposition] + [Какой] + [бы] + [Modified Noun and/or Subject] + [ни] + [Verb] + [Subject (if inverted)].
• Clause Position: The subordinate clause is highly flexible and can be placed before, after, or inside the main clause (e.g., Я всё равно пойду, какая бы ни была погода. - 'I will go anyway, whatever the weather may be.').
• Punctuation: The какой бы ни clause must always be separated from the main clause by a comma (or commas), regardless of whether it comes before, after, or interrupts the main clause.

Additionally, the lexicalized phrase какой бы то ни было (any whatsoever / of any kind) is extremely common and functions as an emphatic modifier. Note that while the particle ни is unstressed in the standard какой бы ни construction, in this set phrase, the stress falls heavily on the ни (pronounced as ни́ было). Crucially, it is a Negative Polarity Item: it cannot be used in simple affirmative sentences and is strictly restricted to negative contexts (with не, нет, ни, без, etc.), conditions, or questions. Unlike the full subordinate clause, this phrase acts as a phrasal modifier and is not separated by commas. This pattern is common in formal writing and educated speech to show that a condition is irrelevant to the main result.