This is the 2073rd most frequent Dutch word.
gebruikten
"Gebruikten" means "used" (past tense of "to use").
In this sentence, 'gebruikten' is the past tense plural form, referring to an action performed by 'zij' (they) in the past.
Zij gebruikten de computers gisteren voor het project.
They used the computers yesterday for the project.
Here, 'gebruikten' is again the past tense plural verb used to describe the collective action of the students in using their books during a specific past timeframe.
Tijdens de les gebruikten de studenten hun boeken.
During the lesson, the students used their books.
'Gebruikten' describes a past tense plural action, questioning the reason behind the choice of the group addressed ('jullie') in using the old system.