"Ella corre en un campo."
Translation:She runs in a field.
It makes sense to "run in" a field with tall grass but if a field is mowed it is essentially running on a surface IMHO. The expression is a lot more common with "in a field". But there are plenty of results in English books for "run on a field". And the difference is much closer with "run in the field" vs "run on the field". https://books.google.com/ngrams/graph?content=run+on+a+field%2Crun+in+a+field https://books.google.com/ngrams/graph?content=run+on+the+field%2Crun+in+the+field