'Столь' versus 'так'

Question

How does the meaning of 'столь' differ from 'так'? Based on the context of some sentences, I am inclined to view 'столь' as close to the meaning of 'настолько'. Am I correct in this thinking? For example: 'Кроме того, Берлиоза охватил необоснованный, но столь сильный страх, что ему захотелось тотчас же бежать с Патриарших без оглядки.'

Also, is 'столь' useful to know by itself (that is, unique in meaning) or just a secondary synonym?

Answer 1

Столь (or настолько) means quantitative "so" (meaning "so much"), and can be replaced with так or такой anywhere it cannot be confused with qualitative or classifying "so" (meaning "in the same way").

This means you should avoid using так when quantitative meaning is not the only possible one.

Compare two sentences:

Я не настолько накрашена, чтобы принимать гостей.

Я не так накрашена, чтобы принимать гостей.

The first sentence unequivocally means "I'm not wearing enough makeup to welcome my guests".

The second one is dubious: it might mean both "I'm not wearing the right makeup" or "I'm not wearing enough makeup". Though this sentence is a little bit ungrammatical if parsed according to the first meaning, this first meaning is what most people would perceive from this sentence.

Answering you second question, yes, столь and настолько are complete synonyms, столь being the grand style form.

Answer 2

Interesting to note that 'столь' in the quote you give cannot be replaced with 'так', but would require 'такой' instead.

The difference in meaning between 'столь' and 'так' does not really exist. Both are adverbs in the use where they are synonyms, but keep in mind that 'столь' (or its full prototype 'настолько') can be used to modify adverbs as well as adjectives, but 'так' turns into 'такой' (такая, такое) with adjectives.

Here are some examples. Italics designate adjectives

Я Вас любил так искренно, так нежно...
Не столь страшен грипп, как его осложнения.
Вы столь забывчивы, сколь незабвенны.
Голос у нее такой тихий, такой нежный, такой невинный...
Опять выглянуло солнышко, да такое весёлое, яркое!

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As Quassnoi points out (thank you!), with shortened adjectives the shortened form of 'такой' ('такая', 'такое') -- 'так' -- is used:

Начался штурм города, и так силён был натиск армии семерых вождей...