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These are both prepositional phrases. Number 1 should be included. Number 2 does not have to be. Below I explain why. A prepositional phrase must consist of a preposition (behind, on, in, under, ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/17015 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/17015 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
These are both prepositional phrases. **Number 1 should be included. Number 2 does not have to be.** Below I explain why. A prepositional phrase **must** consist of a preposition (behind, on, in, under, around, etc.) _and_ an object (i.e. what the object of the sentence is behind, on, in, etc.) Sentence number 1 omits the prepositional object 'us.' You need that object. If you read the sentence by itself, you'll see why. What are the chairs behind? Us? The house? The sunset? What? Sentence number 2 omits the whole phrase, not just the object. "On it" certainly adds explanation, but you can get away without it. **Summary:** You can omit an entire prepositional phrase. You cannot omit just the object of a prepositional phrase. That being said, authors can and do take grammatical liberties sometimes. If you feel the sentence would sound better a different way, write it that way.