2006in china, almost all gammar books say the tag question of 'he must have arrived' is 'hasn't he' and that of 'he must have arrived yesterday' is 'didn't he'. And they say 'mustn't he' is wrongly used.
to me, the focus is clearly on "(have) arrived", not on "must". So 'hasn't he?' seems to be the logical tag.
one could use "mustn't he?" like this: he must go; mustn't he?
thanks a lot.
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