Mehmet´in fincanını kullaniyorum.

I´m using Mehmet´s cup.

Thank you

Correct ! [ Maşallah Inscrutable your Turkish is improving alot ]

Tunci thank you. Allah´a şükür and wonderful people like you.

i learnt that Ali´s car = Ali´nin arabası (s = buffer  letter ) Right ?

in this example Mehmet´ in fınjanını = (Mehmet´s cup )

shouldn´t we say " mehmet´in finjanı instead ?

Thank you !

Yes you are correct in writing

Mehmet´in fıncanı = Mehmet´s cup

however, you also have a verb in the sentence ´kullanmak´ (to use)

When you look up the verb in the onsite dictionary you will see /ı/

written under the verb.

This means the object of the verb takes an accusative suffix.

Just like with sevmek (to love), where we get seni seviyorum (I love you)

and the object also takes the accusative suffix.

So the full sentence becomes

Mehmet´in fıncanı+n (buffer letter) + ı (accusative suffix) kullanıyorum

thus we get

Mehmet´in fincanını kullanıyorum.

There is only one buffer letter in Turkish. And it is "y". "s" is not a buffer letter. It is the part of suffix "sı".

fincan-ı-n-ı   this n is not a buffer also. We call it "pronominal n" It comes after 3. person singular possesive suffix(it is first "ı" here).

Ah!  I see . things sound clear now

thank you