是 connects a subject with a noun
Use 是 shì for identity, category, or a statement that one noun is another noun. It is not normally used before an adjective or ordinary action.
Wǒ shì xuéshēng. — I am a student.
Zhè shì wǒ de shū. — This is my book.
Tā shì wǒ de péngyou. — She is my friend.
有 expresses possession or existence
有 yǒu means “to have” or “there is/are”. Use it when something belongs to, is available to, or exists for a person or place.
- 我有一个问题。 I have a question.
- 这里有咖啡吗？ Is there coffee here?
- 桌子上有一本书。 There is a book on the table.
The negative is 没有 méiyǒu, not 不有: 我没有现金 means “I do not have cash.”
Location uses 在
For a person or object’s location, use 在 zài: 我在家 means “I am at home” and 书在桌子上 means “The book is on the table.” Do not replace 在 with 是 or 有.
To say that something exists in a place, use 有: 家里有两个人 means “There are two people at home.” The choice depends on whether the subject is being located or introduced as something that exists.
A three-question check
- Am I identifying one noun as another noun? Use 是.
- Am I saying someone has something or that something exists? Use 有.
- Am I saying where a person or object is? Use 在.
This distinction also helps with questions: 你是老师吗？ asks whether someone is a teacher, while 你有时间吗？ asks whether they have time.