Structure is ridiculously important in the art and craft of screenwriting. This blog will try to reinforce this message again and again.
When I teach, I always read this essay to the class. It's written by Josh Becker, a screenwriter and sometimes director who lives on the periphery of the Hollywood system. His credits appear somewhat questionable for him to state the things he states in this essay (some of which I take exception to, most most of which I heartily agree with), but the guy is a produced, professional writer/director and really DOES know what he's talking about.
Read the essay. I find a little bit of inspiration each time I do.