• 報告ほうこく — report
• 警告けいこく — warning
• 広告こうこく — advertisement
• 告白こくはく — confession
• 予告よこく — notice, preview
• 忠告ちゅうこく — advice

告 is made up of 口 (mouth) and 牛 (cow), symbolizing a cow being branded or marked — a way of informing others about ownership. This evolved to mean “to inform” or “announce” something publicly or clearly.

先生せんせいに 欠席けっせきを 報告ほうこくしました。
I reported my absence to the teacher.

彼かれは 彼女かのじょに 愛あいを 告白こくはくした。
He confessed his love to her.

告 is read as コク (koku) in all common compounds and as つげる (tsugeru) when used alone in its kun-yomi. The on-yomi appears in many nouns and formal expressions.