"These apples are hard."
Translation:Apel-apel ini keras.
I know apel-apel is plural, but should "apel ini keras" be marked wrong?
Yes, I think that you should reduplicate.
The English sentence is very specific.
It uses a determiner, so it's about specific apples, these apples (not apples in general), and it uses the plural form.
Apel-apel ini keras = These apples are hard.
Apel ini keras = This apple is hard.
I am sure Duolingo uses nonduplicative uses that are both specific and plural. I found the nonduplicative uses very confusing, in some cases both plural and nonplural anwsers where both correct.
Honestly it makes the indo language look a mess, which it is a bit.