Eronder and these er- words are like thereunder and the there- words in English. (Although for some stupid reason they only accept some of the there- words for translation.) And the there- words generally make reference to something previously mentioned. But this is one example sentence, so there is no chance for previous reference.
TL,DR - Eronder has more specificity.
Lopen is a bit of a tricky word. In general in the Netherlands if someone uses lopen they mean to walk (hence in any case where you see lopen you can always translate it as to walk).
However, sports related it can also mean to run e.g.
- Ik loop een marathon - I run a marathon.
Some changes will be made regarding this, and some things already have been changed. Rest assured that our end priority is to provide consistency throughout the course. However, at the moment the main priority is going through all the reports that we currently have (which should fix a lot of issues) and in general develop/improve the course as well as help people with their questions.
As said, and to avoid possible frustration, you can always use to walk as a translation for lopen.