De or het hartstichting?

The correct Dutch article is:

de hartstichting

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

How to use hartstichting

the hartstichtingde hartstichting
this hartstichtingdeze hartstichting
that hartstichtingdie hartstichting
our hartstichtingonze hartstichting
your hartstichtingjouw hartstichting
every hartstichtingelke hartstichting
which hartstichting?welke hartstichting?
a big hartstichting / the big hartstichtingeen grote hartstichting, de grote hartstichting