A sentence is a group of words that expresses a complete thought．It may be a question，a command，an exclamation，or a statement．Because forming good sentences is the essence of communicating effectively，it is important to understand what constitutes a sentence．

句子是词的组合，可以表达完整的思想。句子分为疑问句、祈使句、感叹句、陈述句。因为句子是表达思想的基本语言单位，因此研究一个句子的内部构造非常重要。