All above is understood, thanks so much for your comments.

Just a thought, as I can't find an example; As it is omitted for example я мужчина (я быть мужчина - I "AM" a man), but man is not in the accusative мужчину, even though I guess technically it is the object.

But was just wondering in past and future where it isn't omitted, for example "I was a man"... would it be я был мужчину or мужчина! Is it a verb which disobeys the rules?