不是，就是⋯⋯
Explanation
该结构用于表达非此即彼的两种可能性，强调在两个选项中必然选择其一，排除其他情况。
This structure expresses two mutually exclusive possibilities, emphasizing that one of the two options must be true while excluding other scenarios.
Examples
他不是在办公室，就是在实验室。
He is either in the office or in the laboratory.
这些衣服都不合适，不是太大，就是太小。
These clothes don't fit well; they are either too big or too small.