sy when expecting an answer which might be one or more people, yw/ydy when expecting a particular individual or group to be named in the answer:

• Pwy sy'n gyfrifol am hyn? - Who is responsible for this? (indefinite - could be one or more people)
• Pwy ydy'r un sy'n gyfrifol am hyn? - Who is the one who is responsible for this? (definite - looking for a particular individual to be named)
• Pwy sy'n dal? - Who is tall? (indefinite - could be a number of people)
• Pwy ydy'r tala? - Who is the tallest? (definite - the answer expects the name of a particular person)
• Pwy ydy gŵr Heulwen? - Who is Heulwen's husband? (definite)
• Pwy yma sy wedi priodi? - Who here is married? (indefinite)