Explain how Mercutio and Tybalt function as foils to Romeo, and identify Act 3's street fight as the play's structural turning point.
A foil is a character whose traits contrast with a main character's, sharpening our sense of the main character by showing what they are not. Mercutio is witty, cynical about romantic love, and quick to provoke a fight for its own sake -- everything Romeo, freshly and earnestly in love, is trying not to be. Tybalt is proud, honor-obsessed, and eager for violence in defense of his family's name -- the feud's logic given a single face, which Romeo is now trying to set aside now that he has secretly married into the Capulet family.
Act 3's street fight is the hinge the entire tragedy turns on. Romeo, newly married to Juliet, refuses to duel Tybalt, her cousin -- a refusal that looks like cowardice to everyone watching, since no one else knows about the marriage. Mercutio, offended on his friend's behalf, fights Tybalt in Romeo's place and is killed, cursing both feuding houses with his dying breath. Romeo, enraged, kills Tybalt in revenge and is banished from Verona. A single scene converts a secret marriage into an irreversible catastrophe, and every consequence in Acts 4 and 5 flows directly from these few minutes.
Mastery looks like: They can explain how Mercutio and Tybalt function as foils to Romeo and can trace the exact chain of actions that makes Act 3's fight the play's turning point.
Common stumbles: Treating the fight scene as just "more violence" rather than the specific hinge the whole plot turns on; missing that Romeo's refusal only looks cowardly because of information the audience has that the other characters lack.