There are some usages of dır / dir / dur / dür, and sometimes they become tır / tir / tur /tür according the the final letter of the word they come after. * The most common usage is when completing a sentence which lacks the verb. As in your examples. "Bu kalem-dir" "O doktor-dur"---> if we just say "bu kalem" or "o doktor", they will be incomplete sentences. You can consider dir and dur as the verb TO BE (am, is are). * When we are talking about facts. Example: The world is round ---> Dünya yuvarlaktır. >>> -tır is emphasizing the fact that the world is ROUND. * When we are talking about possibilities. Examples: He is not answering the phone. Then, he mustn't be at home. -----> Telefona cevap vermiyor. O zaman, evde değildir. - The doorbell is ringing. - Oh, it may be cargo. ----> Kapı zili çalıyor. Oh, kargodur. - I guess you know this. ----> Bunu biliyorsundur, herhalde. --- When do we use tır / tir / tur /tür? When the word ends with one of those letter: f, s, t, k, ç, ş, h, p. Examples: - This is a book. ---> Bu bir kitaptır. - This is a tree. ----> Bu bir ağaçtır. - Jack is very honest. ---> Jack çok dürüsttür.

Bu kalemdir. O doktordur.