Surprisingly, Duo rejected "Grandpa sleeps in the opera" - perfectly good UK English (and apparently AmE too).
In Duo's "This/that/the Grandpa...", Grandpa (a term of endearment, i.e. "my/your grandfather") conflicts with this/that/the which indicate a person I/we don't know, and is rather disrespectful of an elderly person who mightn't even have children.
If Duo really wants to teach disrespectful English, further examples are the wife (meaning: my wife), or her indoors (= my stay-at-home wife), both used colloquially in Northern England by pre-emancipation husbands whose marriages have presumably exceeded the Use by date.
[21 Mar 2019 10:03 UTC]
Pretty much the same.
-er is "slurred" into something like -a(r). So adding a small hint of R is perfect but if you aim for a simple A nobody will hear that you are a non native speaker. A proper e+r shows it though.
Wasser /[ˈvasɐ]/, Bäcker /[ˈbɛkɐ]/, Lager /[ˈlaːɡɐ]/, Fahrer /[ˈfaːʀɐ]/, Theater /[teˈʔaːtɐ]/, aber /[ˈaːbɐ]/
Sometimes within words: blechern /[ˈblɛçɐn]/
Or another similar weirdo: Papier /[paˈpiːɐ̯]/
Since you have learned Russian this might help you:
Russisch: unbetontes о: человек [t͡ɕɪlɐˈvʲek] ‚Mensch‘; Москва [mɐsˈkva] ‚Moskau‘
"The grandpa sleeps in the opera" is a really awkward English translation! We just wouldn't say "The grandpa." The only exception would be that there is a character called "The Grandpa" and he sleeps in the opera, as opposed to an old man sleeping during the opera in the audience. Otherwise, the correct translation would be, "Grandpa is sleeping during the opera," or, "Grandpa sleeps in the opera house." I consider German Duolingo as one of the best languages on the site, but this translation is surprisingly lame.
Is the grandpa playing a role of sleeping IN the opera, of is he sleeping AT the opera.... Non sequitur!