The statement, 'Beavers build dams.' is true. Its passive, 'Dams are built by beavers.', is false, for people also build dams. Th problem lies in implicit quantification.

Similarly, the isolated statement, 'Welsh is spoken in the South of England and English in the North.' is false, for it will be understood as meaning that the language of the South is Welsh while the language of the North is English.