"Behind closed doors."
Translation:Bakom stängda dörrar.
If the common gender form ends in -ad, the neuter form will be -at and the plural -ade.
E.g. överraskad, överraskat, överraskade.
If the common gender form ends in -n, the neuter form will be -t, and the plural -na.
E.g. uppäten, uppätet, uppätna
And as here, if it ends in -d, the neuter will end in -t and the plural in -da:
stängd, stängt, stängda
These 6 lines of explanation are great! Thank you! I had the same queries. Why don't you just add them in the "tips & notes" section? Just this short explanation will help a lot of people avoid the confusion with the passive participles. And even if you have more to explain about it, can't you always add "More explanations coming soon" ;)