This is the 471st most frequent Finnish word.
niihin
"Niihin" means "to them" or "into them" in English.
'Niihin' is used here as an illative plural form of 'se', referring to the empty boxes as the destination of the action.
Laitoin kirjat niihin laatikoihin, jotka olivat tyhjiä.
I put the books into those boxes that were empty.
'Niihin' is employed as an allative plural form of 'se', indicating trust in a specific group of people.
Hän luotti niihin ihmisiin, jotka auttoivat häntä.
He trusted those people who helped him.
'Niihin' is used as an illative plural form of 'se', pointing to the specific discussions of interest.