One-point perspective rules
In this video, Vladimir London, a Life Drawing Academy tutor (https://lifedrawing.academy/), will explain the rules of one-point perspective. To learn how to draw in perspective, take the Life Drawing Academy course - https://lifedrawing.academy/free
Here's what you need to know about drawing in one-point perspective:
There is only one vanishing point for an object, hence the name - one-point perspective. In this drawing, one cube is seen from above; therefore it is below the horizon. I will draw another cube at the eye-level. The horizon crosses this cube because the horizon is always located at the eye-level. The frontal plane of a cube is a perfect square in one-point perspective. The back side is a smaller square and all other sides are foreshortened.
Now, let's draw three cubes below, above, and at the horizon level. The same vanishing point will be used for all three cubes. The vertical line will be exactly in the middle of each cube. The frontal side of a cube is seen without any distortions. We draw such a side as a perfect square. Here are three equal size sides. From each corner, the vanishing line goes into the vanishing point. The back side of each cube is also a square. Because it is further away from a viewer, it is smaller. Also, according to the aerial perspective the back outlines are slimmer and the frontal outlines are bolder. As you can see, the cube above the horizon is seen from below, so its bottom plane is visible; while the cube below the horizon is seen from above, and we can see its top side. When a cube is at the horizon level, the only visible side is the frontal one. So, in the one-point perspective, we can only see one or two sides of a cube.
Here's another drawing of three cubes, which are located at the same levels as the previous three cubes. However, the levels of the back sides do not match. I indicate the difference in red here. This happens because the proximity to a viewer is different. So, despite the elevations being the same for each pair of cubes, their sizes are not the same. The cubes on the left are closer to a viewer, and they are smaller in real life than cubes on the right, which are further away, and bigger. There is much more to learn about drawing in perspective in the Life Drawing Academy course - https://lifedrawing.academy/free