What should I say when asked to talk about the dynamics of a piece?

Begin by making a judgement on whether the music sounds piano (quiet) or forte (loud). This gives you a solid baseline on which the answer can be built from. It's okay to fine tune the answer later if you wish.

If the music begins to increase in volume, but does so over the course of the bar or of multiple bars, be sure to write that the music crescendos between (for example) bar 7 and bar 9. Likewise, if it gets quieter, reference a diminuendo. If the sound increase is sudden, explain that the music becomes forte in bar 8, or fortissimo if you think the music is already at a forte level. Likewise, if it suddenly goes quiet, reference a piano in bar 12, or a pianissimo if the music is already piano.

If the changes in dynamics are only slight, then you may wish to reference a mezzo piano or mezzo forte, depending whether it is moderately quiet or moderately loud. There is always a little bit of discrepancy allowed in these answers, as some may perceive forte as fortissimo, and mezzo forte as forte, etc.

Whilst you may not always be required to specify bar numbers, it is a useful skill to be able to consolidate. By doing this, it ensures that you are equipped to answer any question about dynamics, whether is specifies bar numbers or not. Even if you find that it doesn't, try to do it as much as you can, so that your answer is as clear as possible.