是
shì
Connecting nouns with "shi"
是 links two nouns — A is B. It is not used to attach an adjective to a noun.
A + 是 + B
How it works
是 (shì) equates two nouns or noun phrases: 我是学生 (I am a student), 他是我朋友 (he is my friend). Negate it with 不, never 没: 我不是老师.
The trap is that English uses "is" for adjectives too, and Chinese does not. "I am tired" is 我很累, with no 是 anywhere. Reserve 是 for the cases where you could write an equals sign between two nouns.
Examples
wǒ shì xué shēng
我是学生。
I am a student.
tā bú shì wǒ de lǎo shī
他不是我的老师。
He is not my teacher.
zhè shì shén me?
这是什么？
What is this?
Common mistakes
- 是好 is not a sentence. Use 很好 for adjectives and save 是 for noun = noun.