Hello Mingxiang, Whole books have been written on this topic! 'The' is called the definite article and it is used to talk about known things. It is not usually used before proper names like 'Bob' and 'China'. We used 'the' before most bodies of water - rivers, seas, oceans but not lakes. Mountain ranges but not mountains. Groups of islands but not individual islands Things of which there are only one in our daily knowledge - the sun, the moon Something which has already been introduced. Here is a book, the book is red. This is only a very brief answer from a big topic so I hope it helps you. Bob