"That apple is small."
Translation:Şu elma küçük.
I struggled with the order it should go in for a second and then got it right by remembering that the verb goes last and, in this case, small is connected with the verb (i don't know if there is a grammatical term for what I'm saying, sorry). Now I know there is no verb in the literal translation of this sentence, but if there was it would be "is" and small needed to come after that so kucuk had to go last. This makes a lot more sense in my head, so I hope you get what I'm saying and it helps you out.
If kucuk is before the noun, it defines the noun and is an adjective, as in the small apple. If you mean to say the apple "is" small, then it comes after the noun and is used somehow as a verb to mean "is small".
Kucuktur means "is small". So you could also say "su elma kucuktur" to mean "the apple is small" but you can't say "su kucuktur elma".
Su also is used as "this", so "that" would normally be "o".
Duolingo just makes things complicated at early stages, and without some knowledge of the language, most early lessons do nothing but confuse people.
"That apple is small." Translation: Şu elma küçük.
Şu - demonstrative pronoun.
Usage of the pronoun "Şu." The demonstrative pronoun "şu" (this or that) & its case forms are used to show the object which is a little further away.
Şuna bak! (Look at this!) ---> Dative case, singular.
Şunlara bak! (Look at these!) ---> Dative case, plural.