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.
双语例句
例：
Unless you are trying to lose weight to please yourself, it's going to be tough to keep your motivation level high.
除非你减肥是为了让自己开心，否则很难保持积极性。
例：
We cannot understand disease unless we understand the person who has the disease.