The first question in the viva is likely to be simple: it’s something you will be able to respond to. A question that asks about an aspect of your work that is familiar – maybe a big picture, “How did you get started?” or “How would you summarise your research?” Simple to understand and asked by your examiners to help start the viva well.
Sometimes the first question is characterised as easy, in contrast to an expectation that questions get much harder. Is the last question of the viva supposed to be hard? I’m not sure.
The last question could be, “Have you got any questions for us?” Whatever it is, the last question comes after all of the others. Nervousness should hopefully have faded away. You might be tired from the effort, but you could also be more relaxed than you were at the start of the viva.
All questions in the viva could be challenging. All questions could be well within your capability as a researcher. First and last questions deserve the same attention and focus as any other.
Forget easy and hard. Remember who is being asked, what they’re being asked and why they’re being asked.