If you haven’t learned mechanical drawing well, is it difficult to learn mechanical principles?

To be honest, there is little connection between drawing and principles. Mechanical principles mainly talk about theory and some schematic diagrams are included in the books. The only thing that is related to your drawing may be the course design time, but now it seems to be moving in the direction of computer drawing. For two-dimensional development, I recommend you learn CAD. For three-dimensional, take PROE. Also, fluid mechanics is very important if you are majoring in hydraulics. Many subsequent courses will use related theories. If you are another major, it may not matter. It seems that you are Students majoring in hydraulics