"The color of this skirt is not good."

[zengator]

The "corrective text" (the bit that appears over my answer that shows what I did wrong) says I "used the singular 'Rock' here, instead of the plural 'Rocks'." But the plural of Rock is Röcke, and isn't relevant anyway because the English sentence I was asked to translate says "this skirt", not "these skirts".

EDIT: Ahh! Right! Rocks is singular . . . when in the Genitiv. The answer, dieses Rocks (or, I suppose, Rockes) is correct, but the corrective text was wrong. The correction should have stated "did not use the Genitiv, 'Rocks' or 'Rockes'".

Among other very useful bits, http://coerll.utexas.edu/gg/gr/cas_08.html states:

"masculine and neuter nouns receive an -s suffix at the end of the noun. BUT if the masculine or neuter noun is one syllable long, it usually gets an -es. "

[youngestthomas]

That's quite a strange correction. In any case you should have used genitive case, so you should have used "Rocks" (or maybe "Rockes" NOBODY HAS ADDRESSED MY COMMENT!),

[youngestthomas]

So I've learned that if a masculine/neuter word is one syllable you have to add an -es ending, if its more than one syllable you add just an -s. Duolingo accepts "dieses Rockes," but is "dieses Rocks" preferred for some reason?

[arisplus]

Its a good question and I think it should be answered. It may just be the case that Rocks and Rockes are both accepted in deutschspracheworld.

[Turteltaube41]

That is correct according to Wiktionary: http://de.wiktionary.org/wiki/Rock

[cem3246]

Why do you need to add an s to Rock?

[aldittli]

"Rocks" is not plural; it is genitive. In the genitive case, masculine and neuter genders, "s" is added to the noun as well as to the "der" or "ein" word. "dieses Rocks" this skirt.