My Father’s car = Babamın Arabası

babam = my father 
araba = car

babam + ın araba + sı

my father’s car

wrong > babamın araba
right > babamın arabası
annem = my mother, mom
İngilizce = English [the language]

annem + in İngilizcesi

my mother’s English

Annemin İngilizcesi çok iyi = My mother’s English is very good [=My mother speaks very good English]
Tom = Tom
Türkçe = Turkish [the language]

Tom’un Türkçe’si = Tom’s Turkish

Tom’un Türkçesi çok iyi değil = Tom’s Turkish is not very good [= He doesn't speak Turkish well]
Annemin Arapçası = My mother’s Arabic
Babamın İngilizcesi = My father’s English
Muhammed’in dayısı = Muhammed’s uncle
Mehmet’in amcası = Mehmet’s uncle
Osman’ın parası = Osman’s money

Ali = Ali [a boy's name]

Ali’Nin arabası = Ali’s car [because Ali ends in a vowel]

Note : This is probably one of the hardest subjects in Turkish. Therefore, you need to learn it very well.