A1
Saying what something is with 是
How 是 links one noun to another in Chinese, why it is not the equivalent of the English verb “to be”, and why saying 我是累 for “I am tired” is the most common beginner mistake there is.
是 sits between two nouns and says the first one is the second one. Name and occupation, this and that, person and nationality.
我是学生。
Wǒ shì xuéshēng.
I am a student.
他是我朋友。
Tā shì wǒ péngyou.
He is my friend.
这是茶。
Zhè shì chá.
This is tea.
Nothing agrees with anything. 是 does not change for I, you or they, and it does not change for past or future — 我是学生 covers I am a student and, with a time word in front, I was a student.
It is not the English “to be”
This is the trap, and it catches everyone once. English uses am in both I am a student and I am tired. Chinese uses 是 only for the first. An adjective needs no verb at all, because in Chinese an adjective already works like one.
我是累。
我很累。
累 is an adjective, so it does not need 是 — it is the verb of the sentence. See adjectives without 是 for what goes there instead.
The rule to hold on to: 是 goes between two nouns. If what follows is a description rather than a thing, 是 is not what you want.
Saying something is not
Put 不 in front. 是 is a fourth-tone syllable, so 不 shifts to a second tone before it — written 不是, said bú shì.
我不是老师。
Wǒ bú shì lǎoshī.
I’m not a teacher.
这不是我的。
Zhè bú shì wǒ de.
This isn’t mine.
是 on its own means yes
Because there is no word for yes in Chinese, 是 answers a question that asked with 是. It is the closest thing to yes the language has, and it only works for questions built on this pattern.
——你是中国人吗？——是。
— Nǐ shì Zhōngguó rén ma? — Shì.
— Are you Chinese? — I am.