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

They do have interviews when you apply, but they're optional.

当你申请的时候，他们确实会进行面试。不过也是选择性的。

被学问的自由所迷住 - SpeakingMax英语口语达人

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

These are completely optional, not required for the course, so there's no obligation for you guys to get involved in these things, but I'll mention these things as we go along in case anybody would like to get involved in working with professors or graduate students.

这些都是自愿选择的,而不是课程要求,所以并不强制你们都参加,但随着课程的进行,我还会不断提及,便于任何感兴趣的同学,能够与教授或研究生一同参与研究

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选