De or het toneelopening?

The correct Dutch article is:

de toneelopening

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

How to use toneelopening

the toneelopeningde toneelopening
this toneelopeningdeze toneelopening
that toneelopeningdie toneelopening
our toneelopeningonze toneelopening
your toneelopeningjouw toneelopening
every toneelopeningelke toneelopening
which toneelopening?welke toneelopening?
a big toneelopening / the big toneelopeningeen grote toneelopening, de grote toneelopening