The spells are fairly easy to understand when the actors enunciate clearly, which is almost never. I keep watching and thinking, "Uh, guys, the Anglo-Saxons weren't Parselmouths. You know that, right?" ;) The only time I've been able to follow a spell (not quite word for word, 'cause I'm rusty) was in "The Curse of Cornelius Sigan," when Mackenzie Crook (Cedric/Sigan) spoke very clearly. Of course, now I can't remember what he said!