系动词，也称连系动词（Linking verb），是用来辅助主语的动词。它本身有词义，但不能单独用作谓语，其后必须跟表语，构成系表结构说明主语的状况、性质、特征等情况。
在英语中，系动词的功能主要是把表语（名词、形容词、某些副词、非谓词、介词短语、从句）和它的主语联系在一起，说明主语的性质、特征、品行或状态。它有自己的但不完全的词义，不能在句中独立作谓语，必须和后面的标语一起构成句子的谓语。它虽是虚词，但是其用法是复杂，而且不可忽视。
形容词的位置一般有两种：
1。 放在名词的前面，a red flower; careful girls, delicious food， a heavy rain,
2。跟在系动词的后面.
系动词一般有这几种：
I. is ，am ， are ， was, were,
1.He is (系动词)happy.
2.The chair was (系动词) broken.
He is (助动词) playing football happily.
He is (助动词) going to play football happily.
II. feel, *taste, smell ， sound, *look,
1.He felt happy.
2.It tastes good.
3. The flower smells sweat.
4.The music sounds nice.
5.He looks sad.
III.become, *get, *turn, *go, *grow ， seem 。
1.Our city is becoming more and more beautiful
2.The days gets longer.
3.They turn green.
4. If you keep milk for too long, it will go bad.
5.Your son grew taller and taller.
6.I want to grow up quickly.
7.He seems happy.
IV. Keep，
1. Let’s do some sports and keep healthy.
2.Please keep our classroom clean.
3.Please keep the doors open.
4.Please keep the windows closed.
5.He keeps running .
6.Don’t keep me waiting for a long time.
V stay
The most important thing is to stay happy.
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