Whenever an opponent plays a land, if Hidden Stag is an enchantment, Hidden Stag becomes a 3/2 Elk Beast creature.
Whenever you play a land, if Hidden Stag is a creature, Hidden Stag becomes an enchantment.
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.