所以 (suǒ yǐ) is a conjunction meaning 'therefore', 'thus', or 'so' — it introduces a conclusion drawn from previously stated reasons or facts. The character 所 (suǒ) historically functions as a nominalizer, turning verbs into noun-like phrases, while 以 (yǐ) means 'to use' or 'by means of'; together, 所以 literally conveys 'the reason by which [something follows]', evolving into a logical connector for cause-and-effect relationships. It always appears at the beginning of a clause or sentence, linking it to what came before.

This word is neutral in register and widely used in both spoken and written Chinese — from casual conversation to formal essays. Unlike English 'so', 所以 cannot begin a sentence without an explicit or strongly implied preceding cause; omitting the reason makes the sentence feel incomplete or abrupt. It’s commonly paired with 因为 (yīn wèi, 'because') in the pattern '因为…，所以…', but 所以 can also stand alone when the cause is contextually clear.