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WH-clefts and it-clefts have long been claimed to be interchangeable. However, an examination of naturally-occurring discourse demonstrates that this claim is incorrect. The presupposed part (WH-clause) of a WH-cleft represents information that the speaker can assume the hearer is thinking about. In one variety of it-cleft, the presupposed part (that/wH-clause) represents information which the speaker assumes the hearer knows or can deduce, but is not presumably thinking about. In another variety of it-cleft, the presupposed part represents information which the speaker takes to be a known fact, though definitely NOT known to the hearer.*
Ellen F. Prince (Fri,) studied this question.