I do not blame you! It is a horrible Law!
In Zone 1 [Europe] and several other Zones a defender may not ask his partner whether he has any more of the suit led. That is in Law 61B.
Let us suppose that hearts are led, a defender plays a club, and his partner asks "No hearts, partner?". If he has no hearts then no harm, no foul, though the defender should be told not to do so in future. But suppose he has a heart.
Now Law 63B [the Law which causes the trouble] says that the defender must change his club to a heart. The club stays on the table as a major penalty card. Any card played from by declarer or dummy after this revoke may be changed.
Despite the fact that the revoke has been corrected, Law 63B goes on to say that a penalty is assessed as though the revoke was established, so the normal penalty for a revoke is also assessed, one two or no tricks as appropriate.
Of course, with the major penalty card as well, this can on occasion lead to a loss of three tricks, which seems a very harsh penalty indeed.
It has been suggested to the WBF laws Commission that in the new Law book this should be simplified by returning to the 1985 Law, which just said the revoke is established. That is much easier, and can never cost more than two tricks. If you want to find out which country is in which Zone there is a full list at http://blakjak.com/brg_lnkn.htm