"[B]oth classroom teaching and the teaching material itself have their limitations. A teaching material usually arranges its contents with a systematic design, and therefore is unlikely to exhaust the usages of a certain grammatical form in one unit. Generally, the teacher is supposed to explain any language point involved in the lesson and exemplify its usages in different contexts. However, owing to the limited time and language environment in class, the teacher may not be able to provide students with all relevant information. Even if possible, this requirement will largely increase the teacher's workload. Moreover, this will increase students' pressure and language anxiety as well, which may lead to unsuccessful teaching in the end."
A Handbook of Chinese Basic Forms Focused on Communication, page 2