Using the word 是 (shì)
The closest equivalent of the English word “to be” in Chinese is 是 (shì). This is used to link nouns to other nouns.

是 (shì) indicates “to be”
是 is “to be”, mostly for, am, is, are.

Structure
[noun] + 是 + [noun]

Example
我是学生。 wǒ shì xuéshēng. I am a student.
他是老师。 tā shì lǎo shī. He is a teacher.
这是一本书。 zhè shì yī běn shū. This is a book.

Talking about negation of 是 (shì)
The negation of 是 (shì) is 不是 (bú shì)

Structure
[something] + 不是 + [place]

Example
我不是学生。 wǒ bú shì xué shēng. I am not a student.
他不是老师。 tā bú shì lǎo shī. He is not a teacher.
这不是一本书。 zhè bú shì yī běn shū. This is not a book.

You can't use 是 to link adjectives to nouns.
To do that, you have to use 很 (hěn). The basic structure for 很 is the same as for 是, but 很 is used with adjectives.
是 is only used with adjectival verbs or stative verbs for special emphasis, especially contrastive emphasis.

WITHOUT 是	WITH 是
你很瘦。 nǐ hěn shòu. You are skinny.	你是很瘦。 nǐ shì hěn shòu. You really are skinny (despite what you may thought).
他很聪明。 tā hěn cōng míng. He is intelligent.	他是很聪明。 tā shì hěn cōng míng. He really is intelligent (despite what you may think).
那个箱子很重。 nà gè xiāng zi hěn zhòng. That box is heavy.	那个箱子是很重。 nà gè xiāng zi shì hěn zhòng. That box is really heavy (despite what you claim).

是 with a number
When it comes to the word 是 with a number, the 是 can be omitted. But when the object is negated, the word 是 must be there.

Example

Sentence	Changes	Meaning
我弟弟（是）十八岁。 wǒ dì dì (shì) shí bā suì.	👉 我弟弟十八岁。 wǒ dì dì shí bā su	My younger brother is 18 years old.
我弟弟（不是）十八岁。 wǒ dì dì (bú shì) shí bā suì.	👉 我弟弟不是十八岁。 wǒ dì dì bú shì shí bā suì.	My younger brother is not 18 years old.