>Sometimes you see nominative with explicit copulas as well, but I >never had that explained to me as anything other than a "correct" vs. >"incorrect but people say it anyway" distinction. Do we have any >native Russian speakers who can shed more light on that? If you have a narrative set in the past, the sentence "Он был депутатом" with the instrumental is theoretically ambiguous between 'He was an MP (at the point in time of the narrative)' and 'He used to be an MP at some point in his life' (though it will be probably clear in most contexts), while "Он был депутат" can only be interpreted in the former way. This also works with adjectives: A transient state tends to take the instrumental, a permanent one the nominative, e.g. "Она была больная" would rather mean 'She was sickly/sick all the time', while "Она была больной" would mean 'She was sick (at that point in time)'.