я живу в городе - я живу в лесу
Why we use prepositional in "Город" and accusative on "Лес"?
it's not accusative but locative
a small number of words have a seventh case called locative which is different from prepositional and used after в and на
so you say о лесе but в лесу
however о городе - в городе (as город doesn't have this case)