Without a comma after bench , the sentence has the bench eating the sandwich. The comma between sandwich and as is optional. If "as a bus drives by" opened the sentence, it would need a comma following.
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Anonymous Does this sentence have a participle phrase and where (if needed) should the commas go?I am sitting on a bench eating a sandwich as a bus drives by.It's fine without a comma. A comma after "bench" suggests that the passing of the bus and your eating of the sandwich are linked events somehow.