The lovers' split is what kicks off the action and imbalance of the film, ultimately establishing the question(s) of the movie: can these true lovers survive, and can this true love survive?
Act 1 to Act 2 transition is probably the most unclear. To me, it is the arrival and introduction and swordplay of the Masked Swordsman, in pursuit of the kidnapped Princess Bride.
The Midpoint of the film is the re-separation of the lovers after surviving the Fire Swamp. Westley goes to the Pit of Despair for almost the rest of Act 2, and Buttercup returns to the castle.
Act 2 to Act 3 transition is clear as day; when we reach the castle with a redefined team and goal, Act 3 begins and basically takes us to the end of the film, where the wedding is rushed through...
..we settle Inigo's revenge...
..and we answer our core question.
Yes, this true love survives.
Also, you could use this film as a great example of A, B, and C story:
A Story: The Lover's
B Story: "My name is Inigo Montoya. You killed my father. Prepare to die."
C Story: Grandpa and Grandson
That's one of the best B Stories in film history, if you ask me.
Directed by Rob Reiner, written by William Goldman.