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.
subject+不是+Adjective/noun/verb#1+是+Adjective/noun/verb #2
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。
那 不是 一 个 困难，是 一 个 机会。
That isn't a difficulty, it's an opportunity.
