Quoting hanan:

merhaba
may be this question will sound silly but i really can't figure out why we used the fusion constant (Y)in these two examples that i have read in di'li geçmiş zaman.

he was good. o iyiydi.

i was succesful. ben başarılıydım.

i hope u understand my question.
thanx.

Quote:

if the last LETTER is a VOWEL, then you need a buffer which is "y". If it is a CONSONANT, then you don't.

Let's look at the examples:

It was a blue car.

blue: mavi
car: araba
a blue car: mavi bir araba

mavi bir araba+idi ("idi" is the origin of the past tense "di") =

mavi bir arabaydı.

Because the last letter is "a" (a vowel): arabA
arabaYdı.

let's say it is not a vowel:
It was a blue house

blue: mavi
house: ev
a blue house: mavi bir ev

mavi bir ev+idi = mavi bir evdi
Because the last letter is "v" (a consonant): eV

You should memorize the Turkish vowels: a,e,ı,i,o,ö,u,ü