(A changing?) difference between nominative and instrumental case usage in predicates with быть

Question

I would like to know more about the distinction between the use of the instrumental (творительный) and nominative (именительный) case in the context of a predicate (сказуемое) parts of sentences in combination with the (linking?) verb быть.

As far as I understand, there exists a subtle semantic difference between, e.g., мы были друзья versus мы были друзьями. I understood that this difference is mainly temporal: the former case expressing staticity, while the latter

Although I could not easily find any formal rules in academic grammar books (for example in Shvedova, 1980); I found a couple of academic papers that state that there is some 'friction' between the two forms. This makes me wonder whether these forms are sometimes used or interpreted interchangeably (albeit incorrect), and whether this is some sort of ongoing language change or not.

I understand that a few related questions exist already on this stackexchange, however, I am specifically interested in the 'language change'-aspect of it, which was not asked on this stackexchange before.

I would really love to hear answers from the community!

Answer 1

If you can read Russian, I would recommend the article Некоторые особенности согласования в русском языке by Dmitri Shmelyov.

Unfortunately, I only have it in print, but the compilation of author's works is only $2.63 on Google Books, and it's an excellent read.

Here is a brief excerpt from there (translation mine). Note that it concerns adjectives, not nouns:

Use of predicative instrumental with the copula быть in past is a relatively new phenomenon in Russian. Some XIX century authors, as some researches note, do not use it at all. Its main difference from the nominative in similar constructs lies in the fact that the noun's attribute is perceived as transient, relative. This is also related to its usage with so-called semi-content verbs (полузнаменательные глаголы - Q.) such as бывать, становиться, казаться and so on. E.g.: "Тенью от тучи проклубились те дни, и теперь казались ему его искания зряшными и пустыми" // (Шолохов, Тихий Дон); "На березах только что обозначилась молодая зелень, и леса оказались такими большими, такими девственными" // (Пришвин, Календарь природы); "Мокрая свежесть стояла в квартире: с вечера форточка оставалась открытой" // (А. Леонов, Дорога на океан)

Often, the full adjectives are put in instrumental when used with the verbs with the general meaning of movement or presence: "Всю ночь и утро сеял частый осенний дождь, и деревянные домики, насквозь промокшие, стояли серыми и печальными" // (А. Андреев. У окна)

The author also notes the fact that nouns can be put both in nominative and instrumental, but does not explain it.

Unlike adjectives, this pattern for nouns has been a thing in Russian for centuries, and it's a thing in Church Slavonic as well.