Combine both corrections of tunci and gokuyum

Sen kimsin? Seni nereden tanıyorum?

Siz kimsiniz? Sizi nereden tanıyorum?

(Second one is more polite)

Thank you Tunci. Now I can die in peace

If I won´t be dead or something, I´ll post something about this tomorrow, hopefully. But for now, know that we use it after proper names mostly (names of people, animals, places, organizations, etc.).

And AlphaF gave you the right answer for your example already.

Heart is already beating, but the talented is whom can change its rhythm.

or

Heart is already beating, but the one who can change its rhythm has the talent.

Your translation is not wrong. It just needs a small correction.

Mehmet gelmediğinden dolayı, Ahmet´in de gelmemesini ümit ederim.

I see that I was invited here Thank you for your kind invitation. May I offer you all lokum with your coffees as a tradition.

Just a second thought, he might have bought a new fish for his aquarium

In that case: (Bana) yeni bir balığı olduğunu söyledi.

Bana yeni bir çaylağı olduğunu söyledi.

The slang meaning of "fish" is used here which is a newbie, or a rookie. Çaylak is the proper word in Turkish

"elinde kalmak" has a slang meaning in this sentence and it means that the person saying this is angry and he may beat someone up. People say it when they feel confident about their strength and fighting ability