I wish a grammar buff explained to me once and for all the subtleties of it!
Have argued (peacefully) with someone over a card with an image of a child on swings. I was putting a message inside referring to that particular picture and the swing in the picture. So is it 'the child on a swing' or 'this child on the swing' if I'm referring to the swing in the pic and not some general one. Otoh it's a generic-looking swing in the picture. Arghh!