In Turkish, there are two main types of vowel harmony: two-way (küçük ünlü uyumu) and four-way (büyük ünlü uyumu).

This vowel harmony rules play a crucial role in Turkish grammar, especially when forming new words by adding suffixes to word roots. :

1. Two-Way Vowel Harmony:
   • In this system, the vowels are grouped into two categories.
   • The two categories are often referred to as “hard” and “soft” vowels.
   • Hard vowels: a, ı, o, u
   • Soft vowels: e, i, ö, ü
2. Four-Way Vowel Harmony:
   • In this system, the vowels are grouped into four categories.
   • The four categories are:
     • Front unrounded vowels: e, i
     • Front rounded vowels: ö, ü
     • Back unrounded vowels: a, ı
     • Back rounded vowels: o, u

1. Soft Vowel Harmony (e-type):
   • In cases of soft vowel harmony, you must choose between two forms of the suffix: the one that uses “e” or the one that uses “a”.
   • The rule is as follows:
     • If the last vowel in the word is “a,” “ı,” “o,” or “u”: use the “a” form.
     • If the last vowel in the word is “e,” “i,” “ö,” or “ü”: use the “e” form.
   • Examples:
     • ev [a house], ev [to a house]
     • okul [school],
     • okula [to a school]
     • kahve [coffee], kahveye [to coffee]
2. Hard Vowel Harmony (a-type):
   • In cases of hard vowel harmony, the suffix form depends on whether the last vowel in the word is a hard vowel or not.
   • Examples:
     • arkadaş >> arkadaş + ız >> Biz arkadaşız. (“We are friends.”)
     • hasta >> hasta + sınız >> Hastasınız. (“You are ill.”)
     • If the last vowel is “o” or “u,” then the vowel(s) of the suffix is “u.”
     • tok >> tok + um >> Tokum.