Strengthen skills is too basic
I finished the Dutch tree and would like to use the strengthen skills function but I find that despite mastering the more complicated parts of the language that the strengthen skills exercises are too easy. E.g. it asks me to translate "een man = a man" etc. That is pretty boring and not particularly useful at this stage.
Good going! In my experience with the general skill strengthening function (for Spanish) the sentences increase in difficulty after a while. But you can also just do a skill strengthening exercise for a particular lesson or unit which you may have had more difficulty with. Hope this helps!
Use the 'Strengthen' function for each individual 'skill' from the bottom of the tree until the skills, which are too easy for you. After finishing this you will get hardly sentences like "een man = a man", when you use the general strengthen function.
Instead of trying the strengthening skills for the whole tree, you can try strengthening specific skills, by clicking on a skill in the tree and clicking "strengthen [skill name]" on the right. If you do this further down the tree you will practise more difficult sentences. In these more difficult sentences simpler words will also appear. E.g. De man en zijn vrouw willen alles zien will also strengthen man. Once you have correctly entered these words a few more times, the system will know better that you have fully memorised them, which means simple sentences with these words are less likely to appear.
I guess the issue for you is that you finished the tree with very little practice as you are only level 11 (maybe you had some previous knowledge of Dutch?). This means you haven't entered too many sentences yet, which means the system doesn't yet know you have already fully memorised these simpler words.
Edit: what Pentaan said basically. :)