Difference
区别
"I can always tell a graduate class from an undergraduate class," observed the instructor in one of my graduate engineering courses at California State University in Los Angeles.
“研究生班和本科生很容易就能区别开来，”在洛杉矶加利福利亚州立大学给我们研究生上工程学课的老师如此说。
"When I say, 'Good afternoon,' the undergraduates respond.
“我说‘下午好’，本科生们回答说‘下午好’。
"Good afternoon." But the graduate students just write it down."
研究生们则把我说的话记在笔记本上。”

1.tell a graduate class from an undergraduate class
tell from=tell the difference between A and B
从…可看出；通过…判断出
eg: I could tell by his face that he was angry. 我能从他的脸上看出他生气了。
分清…；区别…，把…与…区分开
eg: to tell the real from the false 区别真伪

2.instructor
n.指导者，教师，也有教练，辅导员等的意思。
instructor主要偏向于导师的意思。一般上课的老师都用teacher。

3.respond
不及物动词，需加介词to或者that引导的从句
She never responds to my letter.
He responded that he remained optimistic.

4.write it down
write down是写下的意思。