Why did Helena betray Hermia's trust & what was the result?
Hermia has told Helena that she (Hermia) is going to elope with Lysander. She is doing this to avoid having to marry Demetrius. But Helena loves Demetrius and wants his approval. So she tells him what Hermia is planning. So the reason why she betrays Hermia's trust is because she thinks that will make Demetrius like her (Helena) better.
The result is that Demetrius chases after Hermia and Lysander. That leads to all sorts of chaos because Puck is going to put the love potion on the wrong people at the wrong time. Demetrius and Lysander will both end up in love with Helena, Titania will fall in love with Bottom, and all sorts of confusion will come about because Helena and Hermia, for example, don't know about the love potion.