A common defect of modern art study is that too many students do not know why they draw.
A drawing should be a verdict on the model. Don't confuse a drawing with a map.
Drawing is not following a line on the model, it is drawing your sense of the thing.
Lines are results, do not draw them for themselves.
A Curve does not exist in its full power until contrasted with a straight line.
Keep a bad drawing until by study you have found out why it is bad.
If you think of a school drawing while you work, your drawing will look like one.
Has your drawing the meaning you saw in the model at first?
You will never draw the sense of a thing unless you are feeling it at the time you work.
Count on big lines to express your ideas.
Because we are saturated with life, because we are human, our strongest motive is life, humanity; and the stronger the motive back of the line the stronger, and therefore more beautiful, the line will be.
Realize that a drawing is not a copy. It is a construction in very different materials. A drawing is an invention.