Eddie: Where do you get these sentences? The one you cite makes no sense; the words "past him" are out of place. Here is one way I would say this: He was on the way back when he heard what they were saying.
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A complementizer, as used in linguistics (especially generative grammar), is a syntactic category (part of speech) roughly equivalent to the term subordinating conjunction in traditional grammar. For example, the word that is generally called a complementizer in English sentences l