Today the following sentence came up in a chat with a Russian friend:
Ты был забавным.
Here the adjective is in
dative instrumental case, but I would expect it to be in nominative together with the verb был/быть.
When I asked my friend why she didn't write Ты был забавный, she explained that this was because it was in past tense. But I cannot find this rule anywhere in Russian grammar books.
Can anybody explain the use of
dative instumental in this sentence?
Edit: I mistakenly wrote dative case above. I meant instrumental case.