“保持”在“保持安静”中是一个系动词。

Stay in Stay quiet is a copula.

句子中的“看起来”是一个系动词。

The word seems in the sentence is a copula.

“感觉”在“他感觉很累”中是一个系动词。

Feels in He feels tired is a copula.

他是一个好学生，句中的“是”是系动词。

He is a good student, where is is a copula.

“天空看起来蓝蓝的”中的系动词是“看起来”。

The copula in The sky looks blue is looks.

“似乎”常常在英语句子中用作系动词。

Seem is often used as a copula in English sentences.

在“她变得很高兴”中，“变得”是系动词。

In She becomes very happy, becomes is a copula.

句子“他成为了医生”中使用“成为”作为系动词。

The sentence He became a doctor uses became as a copula.

动词“是”在句子“她是老师”中充当系动词。

The verb is acts as a copula in the sentence She is a teacher.

在句子“他们很高兴”中，“是”起系动词的作用。

In the sentence They are happy, are functions as a copula.