The genitive case suffixes are –(n)ın/in/un/ün. The letter -n- is used only if the last letter of the word is a vowel.
–(n)ın if the last vowel of the word is a or ı.
–(n)in if the last vowel of the word is e o i.
–(n)un if the last vowel of the word is o or u.
–(n)ün if the last vowel of the word is is ö or ü.
There are many (mostly foreign) words that are exceptions to the above rule (a fact not mentioned by many web sites). Some of them take the opposite suffix, some others double their final letter, while others loose their last vowel. In addition many (but not all) words that end in (ç, k, p, t) transform this letter into (c, ğ, b, d):
Unfortunately there is no rule and you have to memorize these exceptions. Fortunately there is a tool to help you find out if a word is an exception or not:
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