Vowel Harmony

Turkish is an agglutinative language, so in Turkish prefixes and suffixes are added instead of using isolated words.

Thus in Turkish, you should pay attention to vowel harmony.

Vowel harmony is simply a process where vowels of the ending match the same class of those in the root word, so that they're "in harmony".

If the last vowel in the word is a, ı, o, or u: the suffix needs to contain -a.
If the last vowel in the word is e, i, ö, or ü: the suffix needs to contain -e.

If the last vowel in the word is e or i: the suffix needs to contain -i.
If the last vowel in the word is a or ı: the suffix needs to contain -ı.
If the last vowel in the word is o or u: the suffix needs to contain -u.
If the last vowel in the word is ö or ü: the suffix needs to contain -ü.