Here are some examples:
* Pwy ydych chi? (Who are you?)
* Pwy sydd yn y tŷ? (Who is in the house?)
* Pwy yw'r athro? (Who is the teacher?)
* Pwy a wnaeth hyn? (Who did this?)
The word pwy can also be used in questions asking "what" or "which" depending on the context.