Learn Turkish: Düzenli egzersiz yaptığı için onun duruşu benimkine kıyasla mükemmel. - Because he does exercise regularly, his posture is great compared to mine.

The postposition kıyasla means compared to and requires the preceding word to take the dative case (-(y)e / -(y)a). To say to mine, we take benim (my) and add the pronoun suffix -ki to get benimki (mine). Finally, we add the dative suffix -e to satisfy the rule for kıyasla.

Why is there an -n- buffer in benimkine?

Whenever you attach a case marker like the dative -e to a word that ends in the pronoun -ki, you must use the pronominal buffer -n-. That is why it becomes benimkine instead of benimkiye.

Why does the sentence use yaptığı için instead of just çünkü o egzersiz yapıyor?

While using çünkü is fine, Turkish frequently builds because clauses using için (for/because of). To attach an action to için, you must use the factual participle -dık / -tık. Here, yap (do) plus -tık plus -ı (his) forms yaptığı. Thus, egzersiz yaptığı için literally translates to for his doing exercise.

How is the word duruş (posture) formed?