Without knowing which excerpt you are referring to, I would say three of the biggest plot developments are:
* Romeo and Juliet meet at her family's party and they kiss and fall in love. This leads Juliet to see if Romeo's intentions are true, which they are, so they get married.
* Tybalt tries to start a fight with Romeo. Romeo being newly married to Tybalt's cousin, tries to avoid it. Mercutio fights in his stead and is killed, Romeo kills Tybalt and then is banished.
* Juliet is supposed to marry Paris, so she pretends to be poisoned which makes Romeo think she's actually dead so he kills Paris and then kills himself. Juliet wakes up, sees him dead, and then kills herself.
If you are going to write this for your answer to the American School, this is not what the question asks.