CONJ-SUBORD
除非；若不；除非在…的时候
You use unless to introduce the only circumstances in which an event you are mentioning will not take place or in which a statement you are making is not true.
例句
Unless you are trying to lose weight to please yourself, it's going to be tough to keep your motivation level high...
除非减肥是为了悦已，否则很难保持积极性。
We cannot understand disease unless we understand the person who has the disease...
若不了解患者我们就不能了解疾病。