I'm motivated to ask this question because of this other question on ELL SE.
It seems to me that that's a prepositional phrase, but I don't see 'once' in any lists of prepositions, and the only dictionary I found listing it as a preposition is Simple Wiktionary.
So, consider this sentence with 'once':
Once I get home, I get hungry.
and compare to this sentence which definitely has a prepositional phrase:
After I get home, I get hungry.
Is 'once' a preposition in that sentence? Is it ever?
Best Answer
Both once and after are being used as subordinate conjunctions in your sentences. Once I get home and After I get home are fragments if the sentence that follows is missing.
I admittedly am not the greatest grammarian here, but I cannot think of a sentence using the word once where it doesn't clearly fit a different part of speech.