"時の経つのは早い。"

I know it means something around "Time passes by quickly", but why does it have the のは right after 経つ? and why the の before it? Isn't it a verb?

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1. The first の expresses that the antecedent is a subject. You can change it to が and say 時が経つのは早い.
2. The second の makes the clause 時の経つ nominal. It means こと. (But we don’t say 時の経つことは早い, FYI.)
3. は in のは shows that the word or phrase before it is a subject. In this sentence the subject is 時の経つの. As stated above, 時の経つの is a noun clause, so it can be a subject of a sentence. 🙂