Structure:

就是 + Adjective

Meaning:

就是 can be used to modify an adjective to mean “really”, or “really that and nothing else”.

Examples:

1. 他 就是 聪明，考试 总是 第 一 名。

    Tā jiù shì cōnɡ mínɡ ， kǎo shì zǒnɡ shì dì yī mínɡ

    He’s just really smart. He’s always gets the best grade on tests.

To express resoluteness

Used before a verb, this usage has the feeling that the speaker is being stubborn and is unwilling to change their mind.
Structure:

就是+focus

Examples:

1. 我 老公 就是 爱 抽烟，他 根本 戒 不了!

    Wǒ lǎo ɡōnɡ jiù shì ài chōu yān ， tā ɡēn běn jiè bù liǎo

    My boss loves to smoke, he simply doesn’t listen to the warnings.