The iMovie '09 workspace is divided into three panes: the Project pane, the Viewer, and the Event pane.
The Event Library, found on the left side of the Event pane, is where iMovie organizes all imported and captured video. Events cannot be edited in iMovie. Rather, you edit videos by adding videos from the Event Library to projects in the Project pane.
Event and Project footage can be previewed in the Viewer by hovering the playhead (the vertical red line connected to your cursor) and pressing the spacebar on your keyboard to start and stop playback.