merhaba to all. My question: ıf "Ahmet´in dağdaki evi" means Ahmet´s house which is on top of the mountain, why the suffix in dağdaki is -ki,
not -sı, dağdası. If I write "Ahmet´in dağdası evi" is this wrong?

ps. the title must be bu ne demek

I am new here.  Özür dilerim!

Hi, I am also a learner, but I will try to explain. Because I´m not sure how much you know I will break down each word.

Ahmet = obviously a name, and the ´in´ possessive suffix (uses vowel harmony) and means belonging to something (Ahmet), and is the same as the English Ahmet´s 

To show what is possessed here, the word ev (house) has a ´i´ suffix to show that it belongs to something/someone. (Evi = his/her house)

So Ahmet´in evi means Ahmet´s house. My apologies if you already knew this.

Now, the for the part you probably wanted explained  

Dağ = mountain

dağda = on/in/at the mountain

Dağdaki = which is on the mountain

Because the word dağdaki is like an adjective and describes the noun (ev), it is located before the noun in the Turkish sentence structure.

Here is a link explaining the suffix ki

Hope this helped you.

i agree with henry.