围绕主谓宾结构的局部膨胀，简单句就变长了。简单句主要成分包括：主语、谓语、宾语与宾补。次要的结构成分包括定语与状语。主要成分缺少了句子语法不成立，次要成分缺少了句子语法成立。
修饰名词的成分叫做定语，定语可以由形容词、介词、非谓语和句子充当；修饰动词、句子的成分叫做状语，状语可以由副词、介词、非谓语和句子充当。
二、拓展句子
定语和状语进入5种简单句结构中，可以大大地丰富句子的信息量，使句子变得生动、耐读。下图呈现了简单句的逐步膨胀过程：
SVO
基干句
We held a discussion.
加定语
We held a heated discussion.
加状语
Last week we held a heated discussion among the teachers and students of our school on the topic of the importance of good manners at school.
上周我们在师生中进行了一次热烈的讨论，话题谈及了在学校良好礼貌的重要性。
三、压缩句子
句子剩下基干结构，意义也就原形毕露了。例如，
Immediately, realizing the seriousness of the whole incident, he moved to a ground floor room with a small waiting-room to look into the matter himself, leaving his second-in-command（副手）to take over all other cases.
压缩句子剩下基干结构：
He moved to a ground floor room. 他搬到一楼的小型等候室。
简单句的扩张与收缩不会改变句型原来的结构，也不会改变原来句子的语法正确性，但是会引起信息总量的增加与减少，从而大大地影响表达的效果。