This is the 296th most frequent Finnish word.
moni
"Moni" means "many" in English.
The word 'moni' here is used as a determiner meaning 'many,' indicating a quantity of people (ihminen).
Moni ihminen pitää kahvista.
Many people like coffee.
Here, 'monia' is in the partitive plural form, connecting with 'asioita' to indicate things that were left unhandled.
Monia asioita jäi vielä käsittelemättä.
Many things were left unaddressed.
In this sentence, 'monia' is again in the partitive plural form, referring to a diverse group (oppilaita) receiving benefit.