What is represented by a verb can be used as the subject of a sentence by putting の after the plain form of the verb. The dictionary form and nai-form are particularly used. の changes the verb phrase into a noun-equivalent. Here, it is used with such words as たいへん(hard;time-consuming), きけん(dangerous), ざんねん(be a pity), and たのしい(enjoyable), which express the speaker's opinion or thoughts.
毎日じゅぎょうの予習をするのは大変です。
每天預習功課是很辛苦的。
りょこうに行けないのはざんねんです。
不能去旅行覺得很可惜。
參照：Lesson 19-8「兄が本を買ったのはしぶやです。」
哥哥買書的地方是澀谷。