Dear Mary: How can I stop my future son-in-law saying ‘must of’

Q. My future son-in-law has been successfully house-trained in the use of upper-middle-class English over the years that he has been walking out with my daughter. However, one bad habit remains. How can I cure him of saying ‘must of’ when he means ‘must have’? He always says ‘of’ very clearly, as though he really means it. I dare not correct him for fear of making him feel inadequate.
—Name and address withheld

A. First disarm him with praise. Find an excuse to praise the fluency and elegance of his conversation, perhaps by comparing him with a less articulate contemporary. Then add, ‘And I don’t think I’ve ever caught you making a grammatical error — except of course for “must of”. But then so many of your generation seem to make that mistake because “must’ve” sounds so like “must of”.’ With his morale boosted by your endorsement of his 99 per cent articulacy rating, he should in future take pains to avoid saying ‘must of’ so that he can attain a 100 per cent rating.