Difficult to teach" has various possible interpretations because of the many meanings of "teach." It includes "difficult to explain to my students" but could also include "difficult to find activities that lead to my students learning rapidly to do this thing consistently." The difficulty for teachers can also arise from our own lack of knowledge. For example, on the whole, no explanation of the English article system has led to my feeling confident that I truly understand how articles are used in context and that I can explain every example that students might find in authentic contexts and might ask me to explain to them