Those phrases do not strike me as completely native, although I understand the meanings. But "impose enthusiasm" is interesting, because the phrase juxtaposes conflicting ideas (impose=outside versus enthusiasm=Inside). Enthusiasm comes from inside a person and cannot be imposed from the outside. Say "course length", not "courses length". Say "we cannot and need not teach every aspect of a subject."

Zhanpeng, Unfortunately neither of them are what a native speaker would write. Let's try to correct them. 1. It is not very clear what you want to say. The main problem is the verb "impose". "Impose enthusiasm" does not make sense. Let's construct the sentence bit by bit. Let's assume that you would like to see the student become more enthusiastic about his studies. This is a good beginning, because you now have a basic core - "enthusiastic about his studies". We have now at least corrected "enthusiasm of studying". Then we have the verb "become", which is useful for you here. So we can try: It is difficult to make a student become more enthusiastic about his studies. Now, we have a sentence which makes sense. You can build it from there. For example, why do you think it is difficult? 2. Let's try correcting this one. Let's try a basic principle first. "The syllabus for this subject for this year is constructed according to the requirements of the Education Bureau for this level, taking into consideration the amount of time allotted to it during this academic year." You may wish to say, "At each particular level, we aim at teaching you what you are supposed to learn at that level. We obviously cannot cover everything there is to learn about the subject." No courses in the world would aim at teaching the students everything there is to learn about a subject. It is common sense. It would be quite stupid to specifically state this kind of manifestly obvious truth, but what else is there that you wish to say? You see, this kind of thinking process clarifies our thought. It is not just the use of English that ee are trying to learn. An even more important question is,"Do I have something meaningful to say?" Please think about it, and come back to let us know how you wish to build on this initial work to express all your thoughts about teaching. Are you writing an essay?

spoken english tends to be simple and easy, but those sentences are completely normal and understandable