Turkish makes use of three buffer letters -n- -y- -s-
This is to keep vowels apart when adding suffixes.

The buffer letter -S-

Only used with the third person suffix -ı -i -u -ü he, she, it

It becomes -si -sı -sü -su when added to a root word which ends in a vowel.

It is not used in any other instance.

Turkish Buffer Letter -S-
Simple NounMeaningSuffixedMeaning
kedi catkedisi
[kedi-si]
his / her cat
şemsiye umbrella şemsiyesi
[şemsiye-si]
his / her umbrella
kapı doorkapısı
[kapı-sı]
his / her / its door
palto overcoatpaltosu
[palto-su]
his /her overcoat
ütü flatironütüsü
[ütü-sü]
his /her iron
araba cararabası
[araba-sı]
his / her car
kuzu lambkuzusu
[kuzu-su]
his / her lamb

Turkish buffer letter -Y-

Buffer letter -Y- is only used with simple nouns

Buffer -Y is used ON PLAIN SIMPLE UNSUFFIXED NOUNS with the direct object Condition suffix -i the

It becomes -yi -yı -yu -yü when added to a root word which ends in a vowel.

kedi catkediyi
[kedi-yi]
the cat [obj.]

banka bankbankayı
[banka-yı]
the bank [obj.]

Buffer -Y is used ON PLAIN SIMPLE UNSUFFIXED NOUNS with the the Movement Toward Condition [dative] -a, -e to, towards

It becomes -ya -ye when added to a root word which ends in a vowel.

kedi catkediye
[kedi-y-e]
to the cat

banka bankbankaya
[banka-y-a]
to the bank

baba fatherbabaya
[baba-y-a]
to the father

The buffer letter -N- with Ownership Condition

Buffer letter -N- is always used for the Ownership condition [genitive] -in -ın -ün -un

It becomes -nin -nın -nün -nun for "owners" that end in a vowel.

Bare nouns in the ownership condition do not use -Y- as a buffer letter.

Turkish Buffer letter -N- Ownership Condition
Bare NounExtended Noun
köprünün
[köprü‑nün]
of the brıdge, the brıdge'sköprüsünün
[köprü‑sü‑nün]
of his brıdge, his brıdge's
masanın
[masa‑nın]
of the table, the table'smasalarının
[masa‑ları‑nın]
of their table, their table's
pencerenin
[pencere‑nin]
of the window, the window'spenceresinin
[pencere‑si‑nin]
of its window, its window's
yolcunun
[yolcu‑nun]
of the traveller, the traveller'syolcularının
[yolcu‑ları‑nın]
of theır traveller, the traveller's

Buffer letter -N- is used with nouns that have already been suffixed.

This shift of buffer letter y to n enables the listener to discern that the word is already agglutinated (extended) by previous suffixes.

Buffer letter -N is used on ALREADY SUFFIXED NOUNS

kedisi his/her catkedisini
[kedi-si-ni]
his/her cat
[3rd person possessed object]

bankası his/her bankbankasını
[banka-sı-nı]
his/her bank
[3rd person possessed object]

kedileri their catkedilerine
[kedi-leri-ne]
to their cat
[3rd person plural possessed motion toward]

The Buffer Letter -N- with -da/-de and -dan/-den

The suffixes -de and -dan are added directly to basic nouns

kedi catkedide
[kedi-de]
on the cat

kediden
[kediden]
from the cat

kapı doorkapıda
[kapı-da]
on / at the door

kapıdan
[kapıdan]
from the door

They become -nda/-nde and -ndan/-nden when added to ALREADY SUFFIXED nouns.

This is to separate them from the extended word:

kedisi his/her catkedisinde
[kedi-si-nde]
on his/her cat

kedisinden
[kedi-si-n-den]
from his/her cat

kapısı his/her doorkapısında
[kapı-sı-nda]
on/at his/her door

kapısından
[kapı-sı-n-dan]
from his/her door

Buffer Letter -n used with Suffixes of Location -nde -nda and Movement -nden -ndan

‑de and ‑den require a buffer ‑n‑ when added to an extended word.

Simple NounExtended Noun
bankadan
[banka‑dan]
from the bankbankasından
[banka‑sı‑ndan]
from his bank
bankada
[banka‑da]
in/at the bankbankasında
[banka‑sı‑nda]
in/at his bank
bankalardan
[banka‑lar‑dan]
from the banksbankalarından
[banka‑ları‑ndan]
from their bank(s)
evden
[ev‑den]
from the house/from homeevinden
[ev‑i‑nden]
from his house/home
evlerde
[ev‑ler‑de]
in the housesevlerinde
[ev‑leri‑nde]
in their houses