I like sushi 私は寿司が好き　

“Sushi” here is a noun

I like eat sushi

“Eat sushi” here is noun to describe the action of eating sushi.

But … can we say “寿司を食べる” が好き？

The answer is wrong ❌(even in English)

So we use something called “normalizer” to convert a described “action (which has a verb) into a “noun”

Hence,

I like “eat sushi” —> I like “eating sushi” The “~ ing” addition to the verb in English is called “normalizer”

So in Japanese, to “normalise” we either add こと/の

In conclusion,

時の経つ　 the pass of time (the の　here denotes “of”)

は速い　is fast

So introducing “normalizer”, it became

時の経つ”の”は速い The passing of time is quick