The word 'the' is a specific, determinative Deictic of a particular kind. It means ʻthe subset in question is identifiable; but this will not tell you how to identify it – the information is somewhere around, where you can recover itʼ. So whereas 'this trains' means ʻyou know which train:- the one near to meʼ, and 'my train' means ʻyou know which train: - the one I ownʼ, 'the train' means simply ʻyou know which trainʼ.
- Halliday (1985), Pag. 161