In Turkish, words generally obey two main vowel harmony rules. Those are called Büyük Ünlü Uyumu (Major Vowel Harmony) and Küçük Ünlü Uyumu (Minor Vowel Harmony). As it is known, Turkish is an agglutinative language. Thats why, a good understanding of these rules is neccessary to use the suffixes properly. 

- Major Vowel Harmony 

As it is known, Turkish Alphabet has 8 vowels. Those vowels can be classificated into two types based on the situation of the tongue, called Kalın Ünlüler (Back Vowels) and İnce Ünlüler (Soft Vowels). There are 4 back vowels and 4 soft vowels.

If the first syllable of the word has a back vowel (a,ı,o,u), other vowels of the word should be back vowels too.

ağaç, adım, ağız, boyun, balık, çocuk, dalga ….

If the first syllable of the word has a soft vowel (e,i,ö,ü), other vowels of the word should be soft vowels too.

eğri, eşek, öküz, üzüm, gözlük, yüzük, tezek …

 - Advanced Notes For Major Vowel Harmony

1- Most Turkish original words obey this rule. But there are some exceptions.

anne, dahi, elma, hangi, hani, inanmak, kardeş, şişman

2- Generally  foreign words dont obey this rule.

            ahenk, badem, ceylan, çiroz, dükkân, fidan, gazete, hamsi, kestane, limon, model, nişasta, pehlivan, selam, tiyatro, viraj, ziyaret

3- Generally united words dont obey this rule.

            açıkgöz (açık+göz), bilgisayar(bilgi+sayar), çekyat(çey+yat), hanımeli(hanım+eli)

4-  Some suffixes (-gil, -ken, -leyin, -mtrak, -yor, -ki) dont obey this rule.

            akşam-leyin, bakla-­gil-ler, çalışır-ken, ekşi-mtırak, yürü-yor , akşam-ki

- Minor Vowel Harmony 

As it is known, Turkish Alphabet has 8 vowels. Those vowels can be classificated into two types based on the situation of the lips, called Düz Ünlüler (Flat Vowels) and Yuvarlak Ünlüler (Round Vowels). There are 4 flat vowels and 4 round vowels.

If the first syllable of the word has a flat vowel (a,e,ı,i), other vowels of the word should be flat vowels too.

anlaşmak, kayıkçı, yelek, bilek, deli, sesli …

If the first syllable of the word has a round vowel (o,ö,u,ü), the first following vowel of the word should be close round vowels (u,ü) or wide flat vowels (a,e). That means “o” and “ö” can only be at the first syllable of a word. (Sure there are some exceptions as horoz)