the official athletic shoes of Andover and Exeter

A friend of mine who is a GMAT instructor puzzled his class the other day with the following grammar question(pick one of each pair):

If I was/were he/him, I would not do it/so.

The (only) correct answer is "If I were he, I would not do so."

There was some major confusion among the non-native speakers over that. This also means that the Nike slogan, "Just do it," is incorrect, but can you imagine the pansy-ass shoes advertised by the phrase "Just do so"?