At the bottom of the page I have put a copy of my Learning Log page for the research exercise of examining the ways van Gogh uses lines and marks in his drawing Wheat field with Sun and Cloud. But I also thought I should look at a range of his drawings to get a good idea of how he draws.
So this, below is Wheat field with Sun and Cloud, 1889. My expansive notes about this one are at the bottom of the page. But I think this is an impressive use of line. I am surprised at the way you can use hatching to draw the sky. Which I would not have imagined possible. It seems like you ought to draw everything constantly (or as often as possible) in order to learn to see the world in lines.
In the image below van Gogh uses lots of curved lines which describe the shapes and movement of the trees, grasses and clouds. The broadness of the lines, as well as the darkness of the ink show the solidity of the tree against the softness of the sky, and the flimsiness of the grass is somewhere between the two. But it is important to remember the effectiveness of the curved lines, when all working together in this way; the illusion of movement is exceptional, incomparable to any artist I have come across.
The lines in the Cypress drawing are more varied than those in Cypresses. There are dots and straight lines, curved lines and circles; fine and broad lines. It is fairly confusing for the eye, chaotic and busy. It is interesting as a way of seeing all the different textures and contours in one landscape.