1

Okay, I'm not sure if they are "rules" or "grammatical", but here they go:

After a while our necks began to ache, so we sat on the rest chairs behind (us). I liked it here. The air was fresh and there was a low hip roof to shelter us from the wind.

To give myself something to do, I searched for constellations. I wasn't an expert (on it); my knowledge was limited to horoscope columns and astronomy documentaries.

As an author, would you omit the bolded parts? Why or why not?

2

These are purely my opinion, but I would say (1) no, I wouldn't omit; and (2) yes, that's fine.

(1) "...we sat on the rest chairs behind."

Behind what? Behind your necks? That's what "behind" is referring to right now, and it doesn't make sense.

(2) "I wasn't an expert."

This sounds natural to me. "Expert" right now is referring to "constellations", which is exactly what you're wanting to refer to.

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2

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.

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