1. We encourage prospective students to sit in on a class in order to see what life is like inside the classroom at Stanford.2. It is disrespectful to sit in on a class where drop-ins are unwelcome, and you can only know if they are welcome by asking the instructor of the class.3. If you wish to audit this course, you'll need to get permission from the course instructor.4. Audit this course for five dollars and have complete access to all the course material, activities, tests, and forums.
 sit in (on a class) audit (a course)sit in on a class = 旁聽一門課，此課通常是實體課程，有實體教室跟設備等等，有可能免費也有可能需要付費sit in 非常常用，可說是 "旁聽" 最常用的英文。audit a course = 旁聽一門課，此課可以是網路課程，也可以是實體課程，有可能免費也有可能需要付費
His family seems happy and united, but it's all a chaade.