The only requirement of hard-pen calligraphy is to write the feeling of soft pen with hard pen, which is why the hard-pen copybooks I recommend to my basic students are Ling Fei Classic, Huang Tingjing, Le Yilun and Declaration Form.
Hard pen calligraphy has two requirements for a single word. The first is the structure. The second is the change of priorities. You can write the structure with any pen, but not every pen can practice the change of priorities.
Because of its unique structure, the artistic pen can write light and heavy changes without much skill and practice. It belongs to the kind of pen that can write, but can't practice writing.
Just like many beginners of writing brush think wolf hair is easier than wool. Because wolf hair is harder and easier to control, the reason is the same.
Hard pen is pencil, pen, gel pen and oil pen from simple to difficult.