from the Advanced Dev Guide (you should find that documentation on Oracle Doc website) :
- Enabling stemming
Stemming is enabled in Developer Studio.
To enable stemming for one or more languages in your project:
1. Open the project in Developer Studio.
2. In the Project Explorer, expand Search Configuration.
3. Double-click Stemming to display the Stemming editor.
4. Check one or more of the language check boxes on the list.
5. Click OK.
To disable stemming, use the above procedure, but uncheck the languages for which you do not want
- Stopwords are defined using ex-Workbench new Experience Manager (when you logg into you application you have a feature dedicated for that)