Which is the most difficult rule of subject-verb agreement in your opinion?

Which lines in this excerpt from act ii of william shakespeare’s romeo and juliet reveal that mercutio thinks romeo would be better off if he stopped thinking about love? mercutio: i will bite thee by the ear for that jest. romeo: nay, good goose, bite not. mercutio: thy wit is a very bitter sweeting it is a most sharp sauce. romeo: and is it not well served in to a sweet goose? mercutio: o here's a wit of cheveril, that stretches from an inch narrow to an ell broad! romeo: i stretch it out for that word 'broad; ' which added to the goose, proves thee far and wide a broad goose. mercutio: why, is not this better now than groaning for love? now art thou sociable, now art thou romeo; now art thou what thou art, by art as well as by nature: for this drivelling love is like a great natural, that runs lolling up and down to hide his bauble in a hole. benvolio: stop there, stop there. mercutio: thou desirest me to stop in my tale against the hair. benvolio: thou wouldst else have made thy tale large. mercutio: o, thou art deceived; i would have made it short: for i was come to the whole depth of my tale; and meant, indeed, to occupy the argument no longer.