They are sure of his doing it. Yes.
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pructusHello...Can this sentence be interpreted to mean the past incicdent, "They are sure that he did it"?I mean, without clear context....They are sure of his doing it.Yes.
pructusThey are sure of his doing it.Taken alone it's a bit ambiguous. To my ear it can mean that they are sure that he is doing it or will do it. To be sure you get the past meaning, use They are sure of his having done it. Either way it sounds overly formal and odd to me.