是

是 ‘to be’ joins two noun phrases and indicates an equational relationship between them.

他是大学生。

王老师是中国人。

他不是大学生。

王老师不是中国人。

 

1. 是 is not used with adjectival verbs or stative verbs.

我的弟弟很高。 NOT 我的弟弟是很高。

他很聪明。 NOT 他是很聪明。

 

2. 是 is only used with adjectival verbs or stative verbs for special emphasis, especially contrastive emphasis.

他是很高！ You really are tall! (despite what you think)

那本书是很贵。 That book is expensive (despite what you claim)

他是很聪明。 He really is intelligent (despite what you may think)

 

3. 是 can be used for contrastive emphasis with action verbs

我是明天走， 不是今天走。I am leaving tomorrow. I’m not leaving today.

 

4. 是 is not used to indicate location or existence

 

5. When the object of 是 includes a number (when it refers to money, age, time, etc)

那本书是 五块钱。

我妹妹是十八岁。

现在是八点钟。

那本书不是 五块钱。

我妹妹不是十八岁。

现在不是八点钟。

 

6. 是 is used to focus on some detail of a situation, for example the time, place or participants in a situation or the material that something is made from.

他是昨天来的。 It was yesterday that he came (he came yesterday)

我是在大学学中文。 It is at university where I study Chinese (I study Chinese at university)

我的耳环是用金子作的。 My earrings are made of gold.