Shuowen Jiezi, China's first dictionary from around 100 CE, reads 是 as 日 (sun) sitting above 正 (straight, correct): the sun crosses the sky on a fixed, unerring path, so whatever matched that straightness was called 是, "correct." Not every scholar agrees the earliest oracle-bone strokes match that reading quite so neatly, but the sense stuck: from "the correct one, this one" the meaning slid gently into the small word every sentence needs today: to be.
A piece of culture
Right and wrong, seen from where you stand In the 4th century BCE, the philosopher Zhuangzi used 是 and its opposite 非 (wrong) to needle certainty itself. In the "Qiwulun" chapter of his book, he argued that what looks 是 from one side of an argument looks 非 from the other, and no one can step fully outside their own view to judge fairly. The compound 是非 (shìfēi) still means "right and wrong" in Chinese today — and, in gossip, simply "trouble."
Use it in a real word
不是
búshì · bat1 si6
"is not" — the tone-shift word already whispered at 不; you already know 不