Adjectives can indeed be plural. If someone asks "Çocuklar nasıl?", you should respond "İyiler" and not "iyi".
The thing I'm confused about is that I thought you can't use plural adjectives if you've given the plural noun (my Turkish teacher told me it's redundant). See the examples below: Çocuklar iyi - correct Çocuklar iyiler - incorrect
But DuoLingo appears to think otherwise
The sentences "Onlar çok iyi and iyiler" are perfectly the same.
Actually you can say "Onlar çok iyidirler." to tell about certain facts (in this context your certain facts or strong assumptions i think). For example, someone asks you about a family you know. And your answer would be like that.
"Onlar iyilerdir." may mean "i think they are fine". In other words it implies prediction. In that context we generally use "herhalde". And with "inşallah" it means "I hope they are fine."
It changes according to the last vowel of the word. If the last syllable of the word has a front vowel (e, i, ö, ü) the vowel(s) of the suffixes will be a front vowel also. And if the last vowel is a back vowel (a, ı, o, u) suffix also will include back vowels.
İyiler, çocuklar, dersler, kızlar...
"Onlar çok iyiler." - They are very good.
I'd like to know the answer to Sky's question as well. If the same sentence was to be applied to items/food and not a person, would there be any difference?
"O" - he,she & it. - Personal pronoun. "Onlar" - they/them - "O" + (n) buffer + "-ler" plural suffix.
Demonstrative pronouns - "İşaret zamirleri."
Bu - This(one) Şu - This(one), that(one) O - That(one) Bunlar - These. Şunlar - These, those. Onlar - Those.
Enlightened note: The plural form of the demonstrative pronouns take the plural suffix "-lar" & the letter "-n" is added before the suffix "-lar."
When to use the demonstrative pronouns?
The three demonstrative pronouns & their derivatives are used to show an object to their distance.
Usage of the pronoun "Bu." The demonstrative pronoun "bu" (this) & its case forms are used to show the closest object. Example: Bunu aldım. (I bought this.) ---> Accusative case, singular. Bunları aldım. (I bought these.) ---> Accusative case, plural.
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. Example: Şuna bak! (Look at this!) ---> Dative case, singular. Şunlara bak! (Look at these!) ---> Dative case, plural.
Usage of the pronoun "O." The pronoun "o" is used for both personal (he, she, it) & as a demonstrative pronoun. The demonstrative pronoun "o" (that) has the same meaning as "şu." Example: Arkadaşım ondan aldı. (My friend bought from that.) ---> Ablative case, singular. Arkadaşım onlardan aldı. (My friend bought from those.) ---> Ablative case, plural.