не столько..., сколько...

The construction не столько..., сколько... is a double coordinating conjunction (двойной/парный сопоставительный союз). It belongs to the exact same grammatical family as не только..., но и..., which is why it follows strict rules for placement and parallelism. It is a sophisticated way to refine your speech in Russian, commonly used in analytical, academic, and formal contexts, though it is also frequent in educated daily speech.

Key Rules:

• Placement: The first part of this correlative conjunction, не столько, must be placed immediately before the first compared element to ensure the logic of the sentence is clear. A frequent error is placing it before the verb when the verb itself is not being compared.
• Parallelism: This conjunction connects однородные члены предложения (homogeneous parts of the sentence). Therefore, the elements following both parts of the conjunction must fulfill the same syntactic role. However, they can be entirely different parts of speech (e.g., comparing an adverb with a prepositional phrase). It is also incredibly common to use this pattern to connect infinitives (e.g., не столько помочь, сколько помешать).
• Case Government (Управление): The main verb or word preceding the conjunction governs both elements, meaning they must both be in the correct case. For example: Она интересуется (чем? - Instrumental) не столько литературой, сколько искусством.
• Prepositions: If the compared elements are the objects of a preposition, the preposition must be repeated after each part of the conjunction (e.g., не столько из-за дождя, сколько из-за усталости). However, if the compared elements are adjectives or participles modifying a noun, the preposition is placed before the noun and is not repeated (e.g., о деньгах не столько реальных, сколько метафизических).
• Connecting Clauses: In addition to connecting parts of a sentence, this conjunction can also connect entire independent clauses in a compound sentence. However, it is vastly more common in everyday and academic Russian to use it to connect homogeneous parts (однородные члены) of a single sentence.
• Punctuation: A comma is placed ONLY before the second part (сколько). You do not put a comma before the first part (не столько) unless a completely separate grammatical structure dictates it (such as an explanatory construction clarifying a preceding general word, e.g., Он любил искусство, не столько литературу, сколько живопись.). Never place a comma between не and столько.
• Frozen Form: As a conjunction, this structure is a frozen, unchangeable form. If the words decline into other cases (e.g., не со столькими людьми, со сколькими...), they are functioning as declinable pronouns or numerals, not as this specific coordinating conjunction.
• Negation: The particle не is always written separately from столько.
• Pronunciation: The words столько and сколько are stressed on their first syllables, while the particle не remains unstressed (it acts as a proclitic).

Nuance vs. Other Patterns:
Unlike a simple negated nominal predicate (not A, but B), this pattern acknowledges that A might be partially true, but B is the defining characteristic. For example, saying someone is 'not so much angry as disappointed' suggests they might be a little angry, but 'disappointed' is the more accurate label.