I usually get to work on foot and then come home on the bus. It refers to a system or service. The universe is nearly 14 billion years old. There's only one. The dove is a symbol of peace in many different cultures. It refers to a whole group of things. I wish I could play the piano. It refers to a musical instrument. Simon is probably the best rugby player in the county. It's with a superlative adjective. Excuse me, where's the bathroom? There's only one in the context. A: I'm reading a great book at the moment. B: Really? What's the book about? It's been mentioned already. https://339efcb25eb75e967b0d-ff8eeadd950d51fd1fc939dca75b3973.ssl.cf1.rackcdn.com/assets_green.swf