The locative case suffixes are –da and –de. The letter -d- is transformed into -t- if the last letter of the word is a strong consonant (ç, f, h, k, p, t, s, ş).
–da if the last vowel of the word is a back vowel (a, ı, o, u)
–de if the last vowel of the word is a front vowel (e, i, ö, 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:
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:
Please enter a word: