a) to separate suffixes
b) to separate consecutive vowels
c) for general language harmonics

And sometimes, all these reasons ‘collide’ in the same buffer letter !!
Note: Buffer letters are only necessary if
the preceding character in the word is a Turkish vowel:
a, e, I, i, o, ö, u, ü.

Buffer letter	Used in what situation?	Turkish Example	Translation
-s-	For indirect 'possession' -- involving pronouns.	karasevdasI	his/her/its hopeless love
-s-	With compound nouns.	bira fabrikasI	beer factory
-n-	For direct possession -- pronouns not involved.	Ayse’nin, arabanIn...	Ayse’s (i.e., something belonging to Ayse), of the car...
-n-	With a word that already has an indirect-possession ending.	ArabasInI kullanIn.	Drive his/her car. (A command...)
-n-	With the pronouns bu, su, o, biri.	Onu yemeyecegim.	I won’t eat that.
-n-	With a –ki ending.	Seninkini yaptIm.	I did yours.
-y-	all other situations	RakIyI içtik.	We drank the rakI.

Based on ideas found in 'Turkish Grammer' and 'Teach Yourself Turkish' by G.L. Lewis
and 'Teach Yourself Turkish' by Asuman and David Pollard...