连系动词是指用来连接主语和表语的动词。它们的句型通常为“主语+连系动词+表语”。一般副词不能当表语，但in, out, up等可以当表语。常用的系动词可分为三类：
- 表示事物特征和状态的系动词，如be, feel, look, seem, sound, taste等。例如：
- She is beautiful. (她很漂亮。)
- The soup tastes salty. (这汤有点咸。)
- He seems happy today. (他今天看起来很开心。)
- 表示由一种状态变为另一种状态的系动词，如go, become, grow, get, turn, fall, come等。例如：
- The weather is getting colder. (天气越来越冷了。)
- He became a doctor after years of hard work. (他经过多年的努力成为了一名医生。)
- The leaves turn yellow in autumn. (叶子在秋天变黄了。)
- 表示保持某种状态的系动词，如keep, stay等。例如：
- Please stay quiet in the library. (请在图书馆保持安静。)
- He kept silent during the meeting. (他在会议期间保持沉默。)
“主语+连系动词+表语”、“主语+及物动词+宾语”、“主语+及物动词+间接宾语+直接宾语”、“主语+及物动词+宾语+补语”、“主语+不及物动词（+状语）“这五个句型合起来称为简单句的五种句型。