Is there a grammatical or syntactical logic behind the use of the instrumental in predicative nouns?

Question

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.

Answer 1

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