Structurally, a sentence is an independent unit which usually contains a number of words to form a complete statement, question or command. Normally a sentence consists of at least a subject and its predicate.从结构上来说，一个句子是一个独立的单元，它通常包含许多单词来构成一个完整的语句、问题或命令。句子通常由主语和谓语组成。