What happens
Dawn breaks on the lovers' one night together. Juliet pretends the birdsong is the nightingale, not the lark, to keep Romeo a little longer, but he must go or die — he climbs down to flee to Mantua, each fearing they have seen the other's living face for the last time. Moments later Lady Capulet brings the "joyful" news of Thursday's wedding to Paris. Juliet refuses. Capulet storms in and erupts, calling her vile names and threatening to throw her into the street unless she obeys. Even the Nurse tells her to forget Romeo and take Paris. Alone and betrayed, Juliet resolves to ask the Friar for help — and if he has none, she has the power to die.
Why it matters
The dawn-parting is one of Shakespeare's loveliest scenes, and it is the last time the lovers are together alive. Their argument over lark and nightingale is a game about stretching time, played by two people the play has given almost none. Every scene after this one keeps them apart.
Capulet's explosion is shocking precisely because we have seen him be reasonable. "Hang thee, young baggage, disobedient wretch" is a father turning on his child the instant she refuses him. The play exposes the violence that was always under the feud's pride, now aimed straight inside the family.
The Nurse's betrayal is the real hinge. Her practical advice — Romeo is banished, marry Paris, he is as good as dead — ends the one adult alliance Juliet had. From this line on she is utterly alone, and her turn to the Friar's potion is the act of a girl with no one left to trust.