In Hamlet by William Shakespeare, Shakespeare uses the guards and Horatio to open the play in order to establish a sense of tension and mystery as well as to introduce some of the central themes of the play before the audience meets the main character, Hamlet.

One of the key ideas that Shakespeare wants the audience to understand before meeting Hamlet is the presence of the supernatural in the play. The opening scene features the appearance of the ghost of Hamlet's father, which sets the stage for the paranormal elements that will be a recurring theme throughout the play.

In addition to the supernatural, the opening scene also establishes the themes of political instability and uncertainty. This theme of political uncertainty will continue to play a major role in the play as Hamlet grapples with questions of legitimacy and power. Finally, the opening scene also introduces the character of Horatio, who serves as a rational, skeptical voice in the play. Horatio serves as a counterbalance to these more irrational elements.

Overall, by using the guards and Horatio to open the play, Shakespeare is able to establish a sense of tension, introduce some of the central themes of the play, and provide a rational, skeptical voice to counteract the more otherworldly and irrational elements that will be introduced later on.