If I had to guess how the basic events go down, in the first episode, the big battle we see in the first trailer leads to a battle between He-Man trying to prevent Skeletor from opening a big portal or unleashing some great evil upon Eternia. He-Man discovers he has the power (heh) to stop Skeletor, but it comes at a cost: splitting the Sword of Power in two. He does so, stopping, and possibly locking away, Skeletor, but at the cost of losing the power and revealing his true identity as Prince Adam to Teela. Furious at him for lying and not entrusting his identity to her, she renounces her allegiance to the prince, going into exile. Then we get a time skip, leading into what the rest of whatever the rest of the show will be.