1. Used to indicate a sole exception to prevent a negative outcome, often paired with 否则 fǒu​zé or 不然 bù​rán to mean "unless... otherwise..."; 2. Used to introduce an exception to a statement made in the main clause, often appearing in the second half of a sentence.

Used to indicate a unique necessary condition, often paired with 才 cái to mean "only if... then..."

(literary) Used like 除了 chú​le to mean "except for" or "other than", indicating that someone or something is the sole exception.