De or het mavoleerling?

The correct Dutch article is:

de mavoleerling

Rule Words ending in -ing: almost always de. See the rule

How to use mavoleerling

the mavoleerlingde mavoleerling
this mavoleerlingdeze mavoleerling
that mavoleerlingdie mavoleerling
our mavoleerlingonze mavoleerling
your mavoleerlingjouw mavoleerling
every mavoleerlingelke mavoleerling
which mavoleerling?welke mavoleerling?
a big mavoleerling / the big mavoleerlingeen grote mavoleerling, de grote mavoleerling