When we walk into a room, our brain perceives it three-dimensionally, seamlessly stitching our visual perceptions together. This means that when you're attempting to render a three-dimensional space in two dimensions, such as on a movie screen, you must pay careful attention to the way in which the audience will perceive the space subconsciously. To avoid this disorienting of your audience and visually fracturing of your narrative, you must follow the most important rule in cinematography: the 180-degree rule.
A new video essay from Wolfcrow shows how to properly adhere to—or break—the 180-degree rule, depending on your intentions with the scene. (Of course, the 180-degree rule is only relevant if there are cuts in the scene; for single-shot scenes, it doesn't apply.) The video covers how to shoot with the 180-degree rule, which bisects the set, for one-person, two-person, three-person, and four-or-more-person shots. It also details some great tips for action scenes and eye line-matching.