In Romeo and Juliet, what does her placing the dagger beside herelf as she drinks the potion signify?
She talks to the dagger, and says
What if this mixture do not work at all?
Shall I be married then to-morrow morning?
No, no: this shall forbid it: lie thou there.
She goes on to wonder if she's going to be in the vault before Romeo arrives, if the friar has poisoned her, if it is a trick so that she will marry Paris... and Paris is totally not what she wants. If the potion does not work, she is willing to commit suicide to prevent this marriage.
I think this moment is symbolic with the dagger because it foreshadows an upcoming situation. Keep that dagger in the back of your head.
In Act IV, Scene 3, Juliet is going to drink the potion that Friar Lawrence has prepared for her. She is worried about the potion for a bunch of reasons.
She worries that it will kill her. She worries that she will wake up too soon and suffocate or wake up too soon and go crazy in the tomb. She also worries that the potion will not work at all and she will have to marry Paris.
When she lays the dagger beside her, she is symbolizing how determined she is not to marry Paris. She is setting the dagger out so that she can kill herself if the potion does not work.