の

The particle 「の」is used to turn a clause into a noun phrase.
Another way to nominalize a clause is with 「こと」. 「こと」refers to the entire content surrounding the event or action expressed by the preceding clause.
Example: 田中さんが旅行に行ったことを知っていますか。(Do you know that Tanaka went on a trip?) The speaker is not just asking about Tanaka's action of "traveling," but also inquiring about details such as where Tanaka went, who they went with, what they did, etc.
In contrast, 「の」refers to a single, specific event or action.
Example: 田中さんが旅行に行ったのを知っていますか。(Do you know that Tanaka went on a trip?) Here, the speaker is specifically asking about Tanaka's action of "going on a trip."

Apart from cases where only 「の」or only 「こと」is appropriate, these two can generally be interchangeable. However, it's important to understand their nuances as mentioned above.
- Some cases where only 「の」is used: + Clauses nominalized before verbs of perception like 見る (to see), 聞く (to hear), 感じる (to feel), 覚える (to remember)...
+ Clauses nominalized before verbs like 止める (to stop), 手伝う (to help), 待つ (to wait)...
+ Emphasizing the subject with the structure 「のは～です」.
- Some cases where only 「こと」is used:
+ Clauses nominalized before です、だ、である (meaning it should be ことです、ことだ、ことである...)
+ Clauses nominalized before verbs like 書く (to write), 伝える (to convey), 話す (to speak)...
+ Clauses nominalized before verbs like 祈る (to pray), 約束する (to promise), 決める (to decide)...
+ Fixed structures using 「こと」.
Additionally, clauses nominalized where the predicate is an adjective like 上手 (good at), 下手 (bad at), 好き (like), 嫌い (dislike), 得意 (skilled at), 苦手 (weak at)... often go well with 「の」.