In A Midsummer Night's Dream, how will the Puck change the situaton of Hermia, Lysander, Helena and Demetrius?
You might find it helpful to focus on one particular scene rather than the whole play to answer this question. Therefore to help you out I am going to focus on Act III scene 2 and look at the changes that occur in the lovers here. Of course, what you need to remember is that previous to this, Lysander has been made to fall in love with Helena instead of Hermia.
The riotous confusion of the Athenian lovers continues in this scene, as Oberon and Puck realise that Puck has made a mistake and has annointed the wrong Athenian youth. We see that Lysander is now desperately in love with Helena, having abandoned his betrothed, Hermia, and we, like Hermia, are bewildered and suspicious of the change in the affections of the apparently steadfast Lysander as he protests his love to Hermia:
Why should you think that I should woo in scorn?
Of course, this situation is further complicated when Demetrius wakes up, having been anointed by Puck, and sees Hermia. Again, he, like Lysander, has a complete change of character in terms of who he loves:
O Helen, goddess, nymph, perfect, divine,
To what my love, shall I compare thine eyne?
This situation seems to escalate as Hermia enters, still trusting in the steadfastness of Lysander's affections, and her resulting horror, rage and anger against Helena who she holds responsible for taking Lysander away from her.
Of course, what is important to note is how Shakespeare is using the events of this play. He is making a comment about love and how fickle our emotions often are, but what is so masterful about what he does is that by making us as the audience laugh at such events, we pronounce our own judgement on ourselves for the irrational acts and words we utter when we are under the spell of love. We are perhaps more fickle when under the spell of love than we give ourselves credit for.