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Usually we don't use a comma before because, except when the comma is a must to clear the meaning of the sentence. But l wonder why Randolph Quirk in his CGEL, page 616, uses a comma before because in this construction.
He was drunk, because he had to support himself on a friend's arm.
Without the comma, the sentence would imply that the man in question became drunk by supporting himself on somebody's arm (rather than by drinking alcohol).
