公开课（open course）是指在互联网上公开提供的课程，这些课程通常是免费的，而且可以让任何人都可以访问和学习。公开课的英文原意是指“公开的课程”，这种课程是为了让更多人可以学习和了解某个主题而设立的。

例句：

I took an open course on machine learning and it was really helpful.

The open course on quantum mechanics was fascinating, but it was also very challenging.

翻译：

我上了一门机器学习的公开课，非常有帮助。

量子力学的公开课很有趣，但也非常难。