的 (de) links a word or phrase to the noun that comes after it. It works like 's in "the teacher's car", and it also joins a description to a noun. It is one of the most common characters in Chinese and a core grammar point of HSK 1.
Owner + 的 + thing.
Put 的 after a person or pronoun to show that something belongs to them. 我的 is "my", 你的 is "your", 他的 is "his" and 老师的 is "the teacher's". To ask "whose?", use 谁的. 的 always has a light, neutral tone.
After a pronoun such as 我, 你, 他 or 我们, 的 is usually dropped before family members, close relationships and places you belong to, such as your school or company. Chinese speakers say 我妈妈, not 我的妈妈, although 我的妈妈 is not wrong.
In other cases, keep 的. 我的手机 is correct; 我手机 is heard in casual speech, but as a learner you are safer with 的.
Description + 的 + noun.
的 also joins a description to a noun. The description always comes before the noun, even when English puts it after: "the apples I bought" is 我买的苹果, literally "I bought 的 apples".
A one-syllable adjective usually goes straight onto the noun without 的: 新手机, 好朋友. When the adjective has 很 in front of it, or the description is longer, you need 的.
If everyone already knows which thing you mean, you can drop the noun and end with 的. 我的 then means "mine", 谁的 means "whose", and 大的 means "the big one".
Learners often forget 的 after 很 + adjective. 很好老师 is wrong; say 很好的老师. Without 很, 好老师 is fine.
The other mistake is putting the description after the noun, as in English. In Chinese, whatever describes the noun, whether it is an owner, an adjective or a whole phrase, comes first, then 的, then the noun.
Practise this grammar point on paper: 20 exercises using only HSK 1 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 1. Paste your own sentences into the level checker to see which words, if any, are above your level.