系动词本身意思不完整，后面必须跟一个“表语”来说明主语怎么样，这就是“主语 + 系动词 + 表语”的系表结构。
常见的四类系动词
1.be 类：am / is / are / was / were
He is a teacher.（他是老师。）The soup is hot.（汤很烫。）
2.感官类（看/听/闻/尝/摸起来）：look、sound、smell、taste、feel
The cake tastes delicious.（这蛋糕尝起来很美味。）
Your idea sounds great.（你的主意听起来很棒。）
The flowers smell sweet.（这些花闻起来很香。）
3.变化类：become、get、turn（变得）
The weather is getting cold.（天变冷了。）
The leaves turn yellow in autumn.（秋天树叶变黄。）
4.保持类：keep、stay（保持）
Please keep quiet in the library.（请在图书馆保持安静。）
最重要的规则：系动词后面跟形容词，不跟副词！
The music sounds beautifully. ✗ → The music sounds beautiful. ✓
She looks happily. ✗ → She looks happy. ✓
后面接名词时要加 like
look like / sound like / taste like / feel like + 名词：
She looks like her mother.（她长得像她妈妈。）
It tastes like chicken.（它尝起来像鸡肉。）
感官系动词一般不用进行时
The soup is tasting good. ✗ → The soup tastes good. ✓
口诀
系表结构少不了，系动词后形容词；后接名词加个 like，感官动词不进行。