A movie is made up of a lot of small narrative parts, the smallest of which is the "beat". If you don't know what the hell that is, what better film to study these cinematic emotional building blocks than with Star Wars.
The storytelling concept of a "beat" was coined by Robert McKee in his book Story, and The Film Theorists do a great job of explaining what it is by using Return of the Jedi as an example. Check out the video below.
Video is no longer available: www.youtube.com/watch?v=TMsE01MTgGU
Now, Return of the Jedi isn't a film that most would think of when it comes to great screenwriting. However, the film does offer beginning screenwriters a good place to start in terms of educating themselves on basic story structure. And what better place to start than learning the building blocks of a narrative?
As it goes, beats make up scenes, scenes make up sequences, and sequences make up entire films. McKee describes a beat as the "smallest unit of change in a script," that means that a beat occurs when a character experiences a major emotional change within the story -- love to hate, happiness to sadness, etc.
This is really helpful to understand, because, if you haven't already noticed, writing a script can get overwhelming and confusing really quickly. But getting organized is definitely possible. Personally, I like to make a beat sheet, which is basically an outline of every beat in your screenplay. (Sometimes I make one after I make an overall outline for the story.) This helps me keep track of every reversal, including where they occur in the story, how they correspond with the story/character arcs, and whether or not they're pushing the story forward.
It has to be said -- Robert McKee isn't the only act in town. There are plenty of other screenwriting theories out there that don't incorporate, or even support, the idea of a beat. However, if McKee's approach works for you, reading up on what beats are is a great place to start learning about story structure.