I want to just go over the requirements of the course that really are required, not the optional piece, just so that you understand what my purpose is pedagogically.

我只想跟大家回顾一下,这门课程的必须要求，并非可以自由选择的,这样你们会理解我的目的何在。

耶鲁公开课 - 1945年后的美国小说课程节选

So they're optional, but they're very, very, very highly encouraged that you do these, because this is going to give you practice for the types of problems that are going to be on the exam.

虽然它们是选做的，但是它们非常非常，非常值得你们去做，因为这对你们来说是一次训练,是用来熟悉考试题型的。

麻省理工公开课 - 化学原理课程节选

So I'm not suggesting that it's an optional problem set that you shouldn't do.

一个可做可不做的问题集,因为不做它你会后悔的。

麻省理工公开课 - 计算机科学及编程导论课程节选