De or het insteekopening?

The correct Dutch article is:

de insteekopening

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

How to use insteekopening

the insteekopeningde insteekopening
this insteekopeningdeze insteekopening
that insteekopeningdie insteekopening
our insteekopeningonze insteekopening
your insteekopeningjouw insteekopening
every insteekopeningelke insteekopening
which insteekopening?welke insteekopening?
a big insteekopening / the big insteekopeningeen grote insteekopening, de grote insteekopening