Objects with straight lines are always the easiest to draw, but what about circles and curves? The first thing you need to know is that circles become ellipses when drawn in perspective. This is no surprise given that squares become diamond shapes in perspective (as shown on the three sides of the cube shown above). Ellipses aren’t ovals, they are a special shape that occur in several places in nature (like the orbits of planets).
Ellipses have two axes of symmetry, the major axis and the minor axis. The key to drawing ellipses properly is to think only about the minor axis. It doesn’t matter what the major axis is doing. Here is the big rule: the minor axis points to a vanishing point. (Nearly everyone gets this wrong, thinking that the major axis needs to be vertical).
Ellipses are really squashed circles. Some can be very squashed and some can be puffy almost like a circle. In fact there are an infinite number of ellipses according to the angle you see the circle at. If you look at a circle at 90 degrees you see a circle; at 45 degrees you see a 45 degree ellipse; at zero degrees you see a line. The second rule for ellipses is: the closer the ellipse is to the horizon line (that is a line between two vanishing pints) the more squashed it becomes.
This means that for a simple object like a cylinder we need two different ellipses to define the top and the base. The ellipse at the top is closer to us so it needs to be bigger than the ellipse at the base which is further away. The ellipse at the top is also closer to the horizon line which means that it needs to be a narrower ellipse. In technical terms the top ellipse has the bigger major axis and the smaller minor axis.
The same principles can be used for vertical ellipses as seen on this quick sketch of a car. Note that the minor axes of both ellipses point to a vanishing point. Also note that the ellipse for the back wheel is smaller because it is further away, and it is narrower because it is closer to a horizon line. In technical terms the front wheel has a larger major and minor axis and the minor axis of the back wheel is more horizontal. If you want to draw something with wheels or circles then it is not a bad idea to start with the circles and get them right first.
When it comes to curves, very often these can be treated as parts of circles. You just have to work out which bit of the ellipse you want to keep.