Simplify the following text for a Grade 7 native English-speaking person.
Original: Perturbed by Hamlet 's continuing deep mourning for his father and his increasingly erratic behavior , Claudius sends for two of Hamlet 's acquaintances â '' Rosencrantz and Guildensternâ '' to find out the cause of Hamlet 's changed behavior .
Simplified: When Hamlet 's father dies , his uncle Claudius becomes king and marries Hamlet 's mother ( Gertrude ) . Hamlet 's father appears as a ghost and tells Hamlet that he was murdered by Claudius .