"Der Mann lernt; er lernt."
The problem with this particular instance is that it's not possible to know if there's a semicolon, if I'm trying to take down a dictation. They don't exactly go "Der Mann lernt - semicolon - er lernt". I think the core of the problem is the specific dialect of the reader, which makes very little - if any - between 'ie' and 'e'. I have often had a problem transcribing 'lese' because it sounds like it's spelled 'liese'. Of course I should know the proper conjugation, but I'm still learning the stuff and I feel that ambiguity is actively detrimental to the process.
The verb sein/to be is a special verb that makes both nouns nominative. If it helps, think of "to be" as an equal sign so it isn't possible for one of the nouns to receive an action. Because "you" and "man" are the same thing, "you" and "man" are both nominative and nominative case for "a man" is "ein Mann."