Where the shape comes from
A mouth 口 with a short stroke inside it — the word on its way out. On the page the only thing separating it from the sun 日 is its build: 曰 is written wider than it is tall, 日 taller than it is wide. It heads 曲, 更, 最, 曾, and both the traditional 書 and 會.
汉字 filed under 曰
No published character on HSK Guide is filed under this radical yet. 汉字库.