The reason why I feel soft when sketching my head is because I didn't draw it on the bone, so it looks soft. The solution is to find some books on human skeleton analysis in the library and understand the characteristics of each plaster head that gives you the first impression. According to this feeling, draw deeper and pay attention to the relationship between sketch and light and shadow, so that progress will be made, and the head will not feel soft again. I hope you can draw patiently. If you don't understand anything, please consult the teacher or good classmates in the class and exchange experiences. You are bound to make progress.