Dummy’s rights
Law 42B2 gives a qualified right of the Dummy to try to prevent any irregularity.
Law 42B3 states that Dummy can only draw attention to an irregularity after play of the hand is concluded.
In what circumstances would a Dummy be able to prevent an irregularity before it occurred?
Comments
The most common is when declarer (or indeed a defender) starts to pull a card out of their hand, dummy can stop them if it's not their lead as long as it's before they have actually led the card.
They can also seek to prevent establishing a revoke by saying "having none" when declarer doesn't follow suit.
I was about to post a separate discussion but let me add it to this one.
If declarer, during play, asks dummy 'how many tricks have we made?', is dummy allowed to answer? Would this be permissible under Law 42 A 2?
In fact, is it proper/permissible for declarer to ask this question?
No Dummy may not answer.
43A1(c): Dummy must not participate in the play, nor may he communicate anything about the play to declarer.
42A2 says that dummy can keep count (e.g. by turning the cards in accordance with 65B), not that he can make known this information. The point is that once play ceases he is no longer dummy, and may then contribute to a discussion about tricks won.
I'm not sure if it's permissible for declarer to ask the question, but dummy definitely shouldn't answer.
I suppose dummy can just shrug his shoulders and look down at the cards of his quitted tricks, effectively telling partner to look there for the answer. :)
One can imagine smiles all round the table if dummy keeps silent with a deadpan face! :)
As with many situation with partner, a simple "You should have been counting" should suffice.