Learn Russian: Этот год пролетел совершенно незаметно. - This year flew by completely unnoticeably.

The base verb is лететь (to fly). The prefix про- adds the meaning of moving "past" or "through" a space. Metaphorically applied to time, пролетел means time "flew by" or elapsed very quickly, emphasizing the completion of this quick movement since it is a perfective verb.

Could I use another verb of motion, like прошёл, in this sentence?

Yes, you can say Этот год прошёл (This year passed), which is the standard metaphorical way to talk about time passing using the base verb идти (to go/walk). Using пролетел instead makes the sentence much more expressive, specifically highlighting how incredibly fast the year felt.

Why do we use the adverb совершенно here instead of a simpler word like очень?

совершенно means "completely" or "absolutely." It pairs much better with незаметно (unnoticeably) than очень (very) does. Because "unnoticeably" describes a total lack of noticing, an adverb expressing absolute completeness fits more naturally than one expressing degree.

What is the root of the word незаметно?