"They are very good."
Translation:Onlar çok iyiler.
Wouldn't it still be corret here with -dir. "They are very good" could be a fact/strong assumption.
-Have you tried the cakes from my favourite pastry shop? They are very good. -En sevdiğim pastanenin pastalarını denedinmi? Onlar çok iyidir.
In this example, because the shop is my favourite, the cakes beeing very good is a fact for me.
With using the pronoun you could either say "iyi" the simple form of adjective or "iyiler" the third person plural form (because of the reference onlar). You can not use iyiyez since there is no reference to biz in this sentence. Now if you do not use onlar you habe to say iyiler and not iyi.