Sometimes you want to explain that an object, concept, or event isn't one thing, but in fact a different thing. This is the perfect structure to convey that in Chinese.

Structure
"不是..., (就)是..." (búshì..., (jiù)shì...) is used to clarify something about a certain noun. What's being clarified can range from adjectives, verbs and even noun phrases.

An easy way to remember this pattern is that it roughly corresponds to the English, "Something is not X, it's Y".

Examples

Nà búshì yī ɡè kùnnɑn , shì yī ɡè jīhuì.
那 不是 一 个 困难,是 一 个 机会。
That isn't a difficulty, it's an opportunity.