In Dutch, the verb koken means "to cook" or "to boil". In English, their is a distinction. Cooking implies a permanent change in the food. I can boil water, and when it cools it becomes cold water again. I can cook food, and when it cools it is still cooked food, not raw. If I heat pasta until it just boils, then turn off the heat, I have boiled pasta - but I have not cooked it.
So when it's vegetables, it's 'boil', but for pasta, it's 'cook'?
If 'kook/koken' means boil and cook, then why does Duo seem to prefer one over the other in many of these exercises? You only ever boil pasta, yet there are many ways to cook vegetables, not just boil. Seems to make very little sense to me