Understanding the Structure "不是，就是..."

The Chinese structure "不是，就是..." is used to express an alternative or a choice between two possibilities. It can be translated as "either... or..." or "not... but..." in English. This structure is particularly useful for indicating that if one condition is not true, then another condition must be true.

In the examples provided:

1. 他不是在办公室，就是在实验室。
   Translation: "He is not in the office; he is in the laboratory."
   Here, the speaker indicates that the person is either in the office or, if not, then in the laboratory.

2. 这些衣服都不合适，不是太大，就是太小。
   Translation: "These clothes are all unsuitable; they are either too big or too small."
   In this case, the speaker emphasizes that the clothes do not fit properly, and the only two options for their unsuitability are that they are either too large or too small.

This structure effectively conveys a sense of exclusivity between the two options, making it clear that one must be true if the other is not. It is commonly used in both spoken and written Chinese to clarify situations where there are limited alternatives.