In his recent book, The Sense of Style, Steven Pinker explains permissible uses of commas and writes this sentence
And when the writer pinpoints the coherence relation he has in mind with an explicit connective such as a coordinator (and, or, *but, yet, so, nor) or a preposition (although, except, if, before, after, because, for), a comma is fine …
(p. 292)
I understand before and after can be prepositions. However, I can't find a reference that because is a preposition. The OED lists senses as an adverb, a conjunction, and a noun. Why has Pinker said that these are prepositions? Pinker is a serious linguist, so I don't regard even the OED's entry as an absolute refutation, but he seems to be on his own here.
Best Answer
On page 310 of The Sense of Style Pinker states:
On page 1011 of The Cambridge Grammar of the English Language Huddleston introduces the lengthy analysis that leads him to designating as prepositions what in traditional grammar would be called conjunctions, among them although, while, after and because:
Co-author Pullum has written an entry for Language Log in which he explains the rationale for regarding because as a preposition. It includes the following extract:
So, in summary, Pinker is following the CGEL in listing although, except, if, before, after, because, for as prepositions.