Gidemeyeceklerdensin

the root: git - go
gidecek - he'll go (t softens to d)
gitMEyecek - he'll NOT go (t restores because of m. ME for negation, y for separating two vowels)
gidEmeyecek - he won't BE ABLE TO go (t again softens. E signals ability)
gidemeyecekLER - THEY won't be able to go (LER for plural)
so far it's a verb, now it switches to a noun:
gidemeyeceklerDEN - FROM those who won't be able to go (DEN is a noun suffix and makes the word a noun)
gidemeyeceklerdenSİN - YOU (are) from those who won't be able to go (finally, the pronoun)
as for the rule, aksios clarifies it for nouns, but i don't know all the rules, do you (for your own language, i mean)? there is a rule or rules, of course, but i don't know them, and i don't encourage you to learn rules. i only encourage you to read or listen without even thinking about how we add the suffixes. trust in your brain, it'll do most of the job by discerning the underlying pattern even without your being aware of it. on the contrary, the more you push it consciously the longer it'll take to realize the pattern.
... and this is just my opinion and i don't think it's a scientific one, but i know it from own experiences