一个语法结构完整的句子至少有一个主语和一个谓语动词；如果是及物动词，还得有一个宾语；如果是系动词，则必须有一个表语或补语：

　　He won.

　　I owe you.

　　John is late.

　　定语和状语虽不是句子中不可缺少的成分，但有助于表达清楚或完整的意思：

　　He won an easy victory.

　　I owe you.

　　John is late again.

　　需要注意的是：一个完整的句子以大写字母开始，以句号结束。写英文时用逗号代替句号、分号、冒号或破折号叫“逗号错”，这正是中国学生所要避免的。请比较下列例句：

　　误：It was raining hard， they could not work in the fields.

　　正：It was raining hard； they could not work in the fields.

　　It was raining hard. They could not work in the fields.

　　It was raining so hard that they could not work in the fields.

　　They could not work in the fields because it was raining hard.

　　It was raining hard， so they could not work in the fields.

　　As it was raining hard， they could not work in the fields.

　　误：The essay is poorly organized， there is no central idea.

　　正：The essay is poorly organized； there is no central idea.

　　The essay is poorly organized： there is no central idea.