The ‘theory’ of Day 15 is that all languages are easy. That they only involve getting used to them. They’re not difficult but different (to the one you’re most familiar with) in degree. In 100 words people reflect on one of their skills. They begin to individualize what they do by choosing to repeat the previous block of text, or continue with the next.
Personally, I enjoy copying and pasting the Spanish that I know on top of the English text. Then I go over that while listening to Spanish. That determines the rate at which I go through HP1.