"This castle is beautiful."
Translation:Dieses Schloss ist schön.
This sentence is only correct if you allow for the improper use of English. "Dieses Schloss ist schön" really translates to "This palace is beautiful." If you wanted to say "This castle is beautiful" you would have to say, "Diese Burg ist schön." Castles are fortifications, palaces are, well, palatial luxuriant residences. They are not interchangeable.
It’s not as clear-cut as you seem to make it out to be, I think.
Windsor Castle and Balmoral Castle are residences of the Queen, for example, yet they are called “castle” and not “palace”.
A residence may be fortified.
Schloss is mostly about being a residence, and may be fortified like a Burg or luxurious like a Palast or Palais.
So um... I'm a little confused as this is the first word I've noticed with a double s. My understanding is that an ß in a word can be substituted with ss. I would have thought therefore that if I saw a double s in a word, it could be replaced with an ß. Like Schloss = Schloß? But there must be some reason they're not teaching us that spelling. Little help?
My understanding is that an ß in a word can be substituted with ss.
As an "emergency replacement" if you can't type the ß, yes. Like how you can replace ä ö ü with ae oe ue if you don't have the vowels with umlauts on your keyboard.
But the proper spelling is the one with ß ä ö ü.
(With the exception of Switzerland, which never uses ß and always writes ss instead.)
I would have thought therefore that if I saw a double s in a word, it could be replaced with an ß.
No, that's not correct.
For example, Waßer is simply wrong.
And Schloß was the correct spelling before the spelling reform of 1996 but is wrong today -- Schloss is how we write it now, because since 1996, ß only comes after long vowels or diphthongs and ss only after short vowels.
Thus das Schloss "the castle" (short o) but der Schoß "the lap" (long o).
Wasser! Of course, one of the first words taught (I also was reminded of good old frisst in a practice lesson after making my comment). I figured I was probably forgetting some double s words I already knew. Thanks for the explanation. I will expect Duolingo to use a double s only where that is the correct and intended spelling, and I only have to be aware when reading comment sections or other places where a double s might actually be a substitute for an ß.
German doesn't have a word burg, and the word schloss means "closed", not "castle".
You can use either of the words Burg or Schloss (capitalised -- they are nouns) here, as long as you use the appropriate gender forms of the demonstrative: Burg is feminine, so diese Burg, while Schloss is neuter, so dieses Schloss.
dieses Burg or diese Schloss would be simply wrong.
If you have a question about why your sentence was not accepted, please always quote the entire sentence that you wrote. Often, the problem is in word order or with the gender of an adjective or article or the like, rather than the word that you think is the problem.