こと

「こと」refers to the entire content surrounding the event/action that the noun or clause before it represents.
Example: 田中さんが旅行に行ったことを知っていますか。The question is not just about Tanaka's action of "going on a trip," but also about the details surrounding it like where, with whom, what they did, etc.
In contrast, 「の」refers to a single, specific event/action.
Example: 田中さんが旅行に行ったのを知っていますか。The question is only about Tanaka's action of "going on a trip."
When nominalizing a clause, except for cases where only 「こと」or only 「の」can be used, these two can basically be interchangeable. However, understanding the nuances of their meanings as mentioned above is necessary.
- Some cases where only 「こと」is used:
+ The nominalized clause comes before です, だ, である... (e.g., ことです、ことだ、ことである...)
+ The nominalized clause comes before verbs like 書く、伝える、話す...
+ The nominalized clause comes before verbs like 祈る、約束する、決める...
+ Fixed structures using 「こと」.
- Some cases where only 「の」is used:
+ The nominalized clause comes before perception verbs like 見る、見える、聞く、聞こえる、感じる、覚える...
+ The nominalized clause comes before verbs like 止める、手伝う、待つ...
+ Emphasizing the subject/topic with the structure 「のは～です」.
- Additionally, clauses where the predicate is an adjective like 上手、下手、好き、嫌い、得意、苦手... often go well with 「の」 as the subject.