The particle “の” is used to turn the preceding clause into a “what” clause in English, as in “what I don’t understand is …”

• 私が分からないのは、なぜ彼が逃げることを選んだかだ -> What I don’t understand is, why he chose to run away
• 私が面白いと思うのは、彼女は美人でもないのにものすごく人気が有るということだ -> What I find interesting is, she is extremely popular even though she is not pretty
• 今あなたが必要としているのは休息だ -> What you need right now is a rest

You can just add “の” to 形容詞 and 形容動詞 as below to create the same structure

• 不思議なのは容器が外から開けられた形跡が無いことだ -> What’s strange is that there is no sign that the container was opened from the outside
• 今大切なのは結果を急がないことだ -> What is important right now is you don’t rush to get a result
• 面白いのはあの二人は性格が正反対なことだ -> What’s interesting is that those two are polar opposites of each other

Note instead of a “what” clause, you can also use “事” as well, just like “thing” in English

• 今大切な事は結果を急がないことだ -> The important thing right now is that you don’t to rush to get a result
• 面白い事にあの二人は性格が正反対だ -> The interesting thing is, those two are polar opposites of each other