Talking about "Hamlet", especially act v, we can say that Gertrude has changed from HAPPY TO REMORSEFUL.
Gertrude has changed since Hamlet's violent confrontation with her. It can be seen that she feels remorse because she has played an important role in the events that provoqued Hamlet's psychological instability and Ophelia's death.
You didn't provide examples, but sound reasoning is when you use logically sound thinking and combine it with factual knowledge. These two combined create a sound argument.
