可以

可以 (kěyǐ) is a modal verb meaning 'can,' 'may,' or 'be permitted to,' with the extra colloquial use of 'okay / not bad.' It expresses three things English speakers should keep apart: (1) permission — '可以进来吗？' 'may I come in?'; (2) possibility/feasibility — '这个问题可以解决' 'this problem can be solved'; (3) evaluative 'okay/passable' — '这家饭店还可以' 'this restaurant is okay.' Two contrasts with English: first, Chinese splits 'can' across three modals — 可以 (allowed, possible), 会 (have the learned ability, future likelihood), and 能 (have the physical capability, situational possibility).

'I can swim' (skill) is 我会游泳; 'I can come tomorrow' (possible/allowed) is 我可以来; 'I can lift this' (physical ability) is 我能搬动. Mixing them up is the single most common HSK 2-3 error. Second, the negative is