This is the 514th most frequent Finnish word.
kertoivat
"Told" (third person plural, past tense).
Here, 'kertoivat' refers to the past action of reporting or informing in a context of news or media, still in the third person plural.
Lehdessä kertoivat sääennusteesta.
In the newspaper, they told about the weather forecast.
In this sentence, 'kertoivat' describes the act of explaining or imparting knowledge about school rules by teachers, again in the past tense and third person plural.
Opettajat kertoivat oppilaille koulun uusista säännöistä.
The teachers informed the students about the new school rules.
Here, 'kertoivat' is the third person plural past tense of the verb 'kertoa', meaning 'they told'. It indicates that the subject, 'he' (they), performed the action of telling something in the past.