[Is there a grammatical or syntactical logic behind the use of the instrumental in predicative nouns?](/questions/6707/is-there-a-grammatical-or-syntactical-logic-behind-the-use-of-the-instrumental-i)
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I know that in some cases the predicate is marked by the instrumental, however i still don't see the logic behind it. Does it fulfill a specific grammatical need, or is it just arbitrary and accepted as is? Funny enough I use it correctly and instinctively, and even notice its absence, when people don't use it where it belongs. I just don't know why. 

Earlier, in the 19th century, the Nominative was more widely used in predicative constructions. In the 20th century the situation changed. There is a number of lexico-semantic clauses show clear case preferences:

Predicate nouns favouring the Nominative case:

* Nouns of nationality
* Evaluative nouns (дурак, красавица, весельчак)
* Semantically bleached nouns, used as modifiers: человек, мужчина, девушка...
* Temporaly unrestricted states: Ведь он был прекрасный учитель!
* The subject in presumed known and the predicate subject contributes little: Он был талантливый студент!
* Pure state, without attention to the current situation and causal relatiionship to other events: Она была красавица, высокомерная и безразличная ко всему.

Predicate nouns favouring the Instrumental case:

* Nouns of occupation, status and function: доктор, руководитель, председатель...
* Deverbal and abstract nouns: занятие, чтение, цель, причина...
* Kinship terms
* Temporaly restricted states: Три года он работал в Самаре учителем.
* The individual in question fits a certain definition: Он был самым талантливым студентом из всей группы.
* Expectations (modal Instrumental): Вот если бы я был американцем!
* Restricting adjunct or modifier: В этой одежде она казалась писаной красавицей.

There is a good article on this topic on the Internet: <http://epubs.surrey.ac.uk/1305/1/fulltext.pdf>