What does it mean?
温故而知新，可以为师矣 (wēn gù ér zhī xīn, kě yǐ wéi shī yǐ) is commonly translated “review the old and know the new; then one can be a teacher.” 温故 means to review or warm up what is already known; 知新 means to gain new understanding.
Where it comes from
The line is from the Analects · 论语, Wei Zheng 2.11. The teaching is about combining memory, reflection and new insight rather than replacing old knowledge with new information.
How to use it
It can be used for study notes, a classroom motto or a reminder to revisit earlier material. The full line makes the learning principle clearer; the shorter 温故知新 is also common, but it omits the phrase about teaching.
What it does not mean
It does not give the reader a formal teaching qualification. It also does not mean that older information is automatically correct or that novelty is always improvement.
Common mistakes
- Treating the saying as a teaching licence.
- Assuming old knowledge is always superior.
- Using only the idiom without explaining the context.