1. Explain it simply
是 (shì) means 'to be' and is used to link a subject with a noun. It's one of the most important words in Chinese.
Use 是 when you want to say someone or something IS something else. Think of it as an equals sign: Subject 是 Noun.
2. Show examples
我是学生。
Wǒ shì xuésheng.
I am a student.
I = student
这是书。
Zhè shì shū.
This is a book.
This = book
他是老师。
Tā shì lǎoshī.
He is a teacher.
He = teacher
3. Share a tip
是 (shì) is only used with nouns. When describing qualities or states (like 'I am tired' or 'She is pretty'), Chinese uses different patterns - you don't use 是 for adjectives.