My opinion, student don’t understand lesson as a result of both. First reason is ‘Student doesn’t intend to learn’. This reason is the most important of students. Some students think she can learn by herself and she has many things more interesting than her lesson in classroom. Second reason is teacher is the bad teach. Sometimes student intent to learn so much but teacher teach lesson too fast. Some teachers don’t teach student. He gave papers and told his student can read lessons by yourself.
So, I think, student can understand lesson, it has two things make its. Student should to intend to learn and teacher should intend to teach. Do you understand your lesson? Because of you or teacher.