De or het mondopening?

The correct Dutch article is:

de mondopening

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

How to use mondopening

the mondopeningde mondopening
this mondopeningdeze mondopening
that mondopeningdie mondopening
our mondopeningonze mondopening
your mondopeningjouw mondopening
every mondopeningelke mondopening
which mondopening?welke mondopening?
a big mondopening / the big mondopeningeen grote mondopening, de grote mondopening