Marking possessive of plural nouns in English
In the Genetive section, there are sentences like this one: Das ist das Haus meiner Großeltern.
To translate that into more idiomatic English, one would write: That is my grandparents' house.
However, if you use the correct form of plural possessive for nouns that take an s to show plurality (grandparents'), it gets marked as being a typo and that one should write grandparents.
This is something the moderators need to fix.